JP5311180B2 - Fixing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing unit of an electromagnetic induction heating system and an image forming apparatus in which an effect of suppressing electromagnetic induction heating of the heating member by an exciting coil or an effect of promoting the electromagnetic induction heating in the heating member by the exciting coil can be controlled. <P>SOLUTION: The fixing unit is provided with the exciting coil which is arranged along the heating member for carrying out electromagnetic induction heating of the heating member by generating an alternating magnetic flux, a sub-induction coil for generating the alternating magnetic flux by going around one part of the alternating magnetic flux which is generated by the exciting coil and an electrostatic capacity switching means for switching the generating direction of the alternating magnetic flux of the sub-induction coil. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a fixing device and an image forming apparatus, and more particularly to a fixing device that heats a fixing roller by an electromagnetic induction heating method and fixes an image on a recording medium by heat generated by the fixing roller.

  A copying machine, a printer, a FAX, and the like have a process of forming an image on a recording member such as plain paper or OHP. Various image recording methods have been realized as a process of forming an image on a recording member. Among them, the electrophotographic method is widely adopted in the above-mentioned devices because of high speed, image quality, cost, etc. It is. In the development and transfer section of image forming apparatuses such as electrophotographic copying machines, printers, fax machines, etc., a latent image is formed from image information, which is electronic information or optical information, and a thermoplastic resin containing pigment is used. The toner (developer) is developed and transferred onto the recording member by a direct method or an indirect (transfer) method to obtain a toner image. The fixing device is a device that heat-fixes the toner image transferred onto the recording member as a permanently fixed image on the recording member. As a fixing method of such a fixing device, the heat roller method is currently most frequently used from the viewpoint of high speed and safety.

  In the heat roller method, a heating roller heated by a heat source (hereinafter referred to as a fixing roller) and a pressure roller disposed opposite to the heating roller are pressed to form a mutual pressure contact portion called a nip portion. In this method, the sheet-like recording member is passed and the toner on the recording member is heated and fixed. Generally, a halogen lamp is used as a heat source, the halogen lamp is built in the fixing roller, the fixing roller is heated from the inside, and the surface temperature of the fixing roller is heated to an appropriate temperature for fixing. However, the heat roller system using a halogen lamp has a problem that there is a limit in reducing the heat capacity (thinning) of the fixing roller and that the start-up time is slow due to the slow start-up of the halogen heater itself.

  Therefore, in recent years, the use of an electromagnetic induction heating method has been studied as a means for heating the fixing roller. In this method, electromagnetic induction heating is performed by generating an eddy current by an alternating magnetic field generated by a magnetic field generating means on a fixing roller having a conductive layer. Since this electromagnetic induction heating method can directly heat the surface layer of the fixing roller to be heated, the rise of heating is faster than the halogen heater, and the waiting time at the start of operation can be shortened. Further, since the heat supply rate is fast, it is possible to follow high-speed operation of the image forming apparatus.

  For example, according to Patent Document 1, a fixing roller by an electromagnetic induction heating method is sequentially supported from the inside to the outside, a support layer (core metal), a sponge layer (foam layer), an electromagnetic induction exothermic layer, an elastic layer, a release layer. Thus, the amount of heat generated in the heat generating layer is insulated by the sponge layer, and the elastic layer and the release layer on the surface side of the fixing roller can be quickly heated. With this configuration, the surface of the fixing roller quickly reaches the temperature required for fixing, and even when heat is deprived of a recording medium such as paper, the supply of heat is performed quickly, which is faster than using a halogen lamp. Driving is possible.

  The electromagnetic induction heating method has a problem that it is difficult to control the temperature distribution in the longitudinal direction of the fixing roller, like the belt heating method, because the electromagnetic induction heat generating layer as a heat generating member is thin. In the fixing device, when a small-sized recording medium is continuously fixed, an excessive temperature rise may occur in a part or all of the fixing roller. A general image forming apparatus is configured to form an image on several types of recording media having different sizes in the width direction. Here, the recording media having different sizes in the width direction include recording media of irregular sizes in addition to recording media of various regular sizes in the A and B rows of JIS dimensions. Even in the case of recording media of the same size (for example, A4 size), the width direction is different between when the longitudinal direction is the transport direction and when the short direction (the direction perpendicular to the longitudinal direction) is the transport direction. You are dealing with recording media of different sizes. When fixing such a recording medium having a different size in the width direction with the fixing device, the heat distribution in the width direction of the fixing member varies depending on the size in the width direction of the recording medium, resulting in temperature unevenness. was there. For example, when a recording medium having a small width direction size is passed and fixed, a large amount of heat is taken away at the position of the fixing roller (paper passing area) corresponding to the width direction size of the recording medium. The fixing temperature is lower than (non-sheet passing area). Such a phenomenon becomes particularly prominent when a recording medium having a small size in the width direction is continuously fed.

  Therefore, if it is attempted to control the fixing temperature in the entire width direction of the fixing roller with reference to the fixing temperature in the width direction central portion of the fixing roller, which is always a sheet passing area, the fixing temperature in the center portion in the width direction of the fixing roller is a desired temperature However, the fixing temperature at both ends in the width direction is increased (overheated). As described above, when a large recording medium in the width direction is fixed in a state where the fixing temperature at both ends in the width direction of the fixing roller is increased, a hot offset occurs on the recording medium corresponding to the temperature increase position. Furthermore, when the fixing temperature at both ends in the width direction exceeds the heat resistance temperature of the fixing roller, the fixing roller may be thermally damaged. On the other hand, if it is attempted to control the fixing temperature in the entire width direction of the fixing member based on the fixing temperature at both ends in the width direction of the fixing roller, the fixing temperature at both ends in the width direction of the fixing roller can be controlled to a desired temperature. However, the fixing temperature at the center in the width direction is lowered. As described above, when the recording medium is fixed in a state where the fixing temperature at the center portion in the width direction of the fixing member is lowered, a cold offset occurs on the recording medium corresponding to the temperature lowered position.

  To solve this problem, in a fixing device using a conventional halogen heater as a heating source, the temperature of the fixing roller is controlled by using a plurality of heaters divided into a central portion and an end portion, respectively. Control has been done. However, in the electromagnetic induction heating method in which the object is heated by the magnetic flux generated from the coil, the method of separately arranging the coil for heating the center part and the coil for heating the end part, such as a halogen heater, increases the cost of the apparatus and the coil. There were many problems such as mutual interference, which was difficult in practice.

  Therefore, methods using a degaussing secondary coil are proposed in Patent Documents 2 to 4. In addition to the exciting coil that performs electromagnetic induction heating, a “demagnetizing secondary coil” is installed at a position corresponding to the non-sheet passing region, and the degaussing secondary coil generated by the magnetic flux change generated by the exciting coil is induced. This is a method of preventing excessive temperature rise by reducing the magnetic flux in the non-sheet passing region by electromotive force and induced current. When suppressing heat generation, a current is generated by using a relay and a switching circuit such as an FET or IGBT, with the secondary coil portion for demagnetization as a closed circuit. When the heat generation is not suppressed, the demagnetizing coil is opened by the switching circuit and does not function as a coil so that no demagnetizing magnetic flux is generated. Heat generation is controlled by opening and closing this switch.

More specifically, a magnetic core extending in the width direction of the sheet and divided into three parts inside the heating roller, and an excitation coil wound in layers around the magnetic core along the inside of the heating roller. A demagnetizing coil (cancelling coil) that is wound around both ends of the magnetic core extending in a direction perpendicular to the excitation coil layer is provided. When fixing the maximum width sheet (recording member), the demagnetizing coil is opened by the switching circuit and does not function. Therefore, it is possible to satisfactorily fix the entire area of the maximum width sheet. When fixing a sheet having a width smaller than the maximum width, the demagnetizing coil is closed by the switching circuit. As a result, at the end of the heating roller with respect to the width direction of the sheet, the magnetic flux change caused by the exciting coil causes not only the induced current (eddy current) in the heating roller but also the counter electromotive force (the accompanying current) in the demagnetizing coil. . Therefore, the temperature rise at the end of the heating roller can be suppressed.
JP 2000-214702 A JP 2001-60490 A JP 2001-135470 A Patent No. 3,933,116

  However, according to the heat roller type induction heating and fixing using a demagnetizing coil as in Patent Documents 2 to 4, a leakage flux is inevitable because there is a space in the magnetic flux path between the exciting coil and the demagnetizing coil. A degaussing effect cannot be obtained. In order to increase the demagnetizing effect, there are methods of increasing the number of turns of the demagnetizing coil or enlarging the demagnetizing coil itself, but the heating device becomes large. Furthermore, there is a method of increasing the magnetic coupling by installing a core in the magnetic flux path of the exciting coil and the degaussing coil, but the structure of the heating device becomes complicated.

The present invention has been made to solve these problems, to provide a fixing device and an image forming apparatus of an electromagnetic induction heating can be controlled electromagnetic induction heating method to suppress effect of the heating member by the excitation coil Objective.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たす前記コンデンサに切り替えることに特徴がある。よって、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を制御可能な電磁誘導加熱方式の定着装置を提供することができる。 In order to solve the above-mentioned problems, the fixing device of the present invention is arranged so as to be along the heating member using the heating member to heat and fix the image on the recording member while nipping and conveying the recording member in the image forming apparatus. An excitation coil that generates an alternating magnetic flux and electromagnetically heats the heating member, a demagnetization coil that circulates a part of the alternating magnetic flux generated by the excitation coil and generates an alternating magnetic flux, and is connected to the demagnetization coil and is different Capacitance switching means for switching to one of a plurality of capacitors having capacitance, and the capacitance C of the capacitor is changed by the capacitance switching means.

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, it is possible to provide an electromagnetic induction heating type fixing device capable of controlling the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たすコンデンサに切り替える。よって、副誘導コイルに、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を持たせることで、非通紙部昇温を確実に防止できる。 Further, the capacitance C of the capacitor is changed by the capacitance switching means.

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, by giving the sub induction coil the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil, it is possible to reliably prevent the temperature rise of the non-sheet passing portion.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たすコンデンサに切り替える。よって、副誘導コイルに、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を持たせることで、非通紙部昇温を確実に防止できる。 Further, the capacitance C of the capacitor is changed by the capacitance switching means.

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, by giving the sub induction coil the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil, it is possible to reliably prevent the temperature rise of the non-sheet passing portion.

Further, according to the material of the recording member, the lateral width of the recording member, or the image area width on the recording member, the capacitance is switched to one of the capacitors by the electrostatic capacity switching means.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たすように可変することに特徴がある。よって、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を制御可能な電磁誘導加熱方式の定着装置を提供することができる。 The fixing device of the present invention is arranged along the heating member to generate an alternating magnetic flux in the fixing device that heats and fixes the image on the recording member using the heating member while nipping and conveying the recording member in the image forming apparatus. The capacitance of the exciting coil that electromagnetically heats the heating member, the demagnetizing coil that circulates a part of the alternating magnetic flux generated by the exciting coil and generates the alternating magnetic flux, and the variable capacitor connected to the demagnetizing coil is variable. Electrostatic capacity variable means, and by the electrostatic capacity variable means, the electrostatic capacity C of the variable capacitor is

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, it is possible to provide an electromagnetic induction heating type fixing device capable of controlling the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たすように可変する。よって、副誘導コイルに、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を持たせることで、非通紙部昇温を確実に防止できる。 Further, the capacitance C of the variable capacitor is changed by the capacitance changing means.

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, by giving the sub induction coil the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil, it is possible to reliably prevent the temperature rise of the non-sheet passing portion.

（但し、ｆは励磁コイルの交番磁界の周波数、Ｌは消磁コイルのインダクタンス、πは円周率である。）を満たすように可変する。よって、副誘導コイルに、励磁コイルによる加熱部材の電磁誘導加熱を抑制する効果を持たせることで、非通紙部昇温を確実に防止できる。 Furthermore, the capacitance C of the variable capacitor is changed by the capacitance changing means.

(Where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). Therefore, by giving the sub induction coil the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil, it is possible to reliably prevent the temperature rise of the non-sheet passing portion.

Further, the capacitance of the variable capacitor is varied by the capacitance varying means according to the material of the recording member, the lateral width of the recording member, or the image area width on the recording member . Therefore, even when recording members having different widths are discontinuously passed, or even when recording members having different image area width images on the recording member are passed, it is possible to reliably prevent overheating of the end portion, In addition, it is possible to appropriately control the fixing temperature distribution according to the lateral width of the recording member or the image area width on the recording member.

  Further, an image forming apparatus as another invention includes the above fixing device and is characterized in that an image is formed on a recording member. Therefore, fixing efficiency can be improved, and high-quality image formation is possible.

According to the fixing device of the present invention, the capacitance of the capacitor connected to the degaussing coil is set, the impedance of the auxiliary induction circuit composed of the exciting coil, the degaussing coil, and the capacitor is lowered, and a large current flows through the degaussing coil. Thus, the effect of suppressing the electromagnetic induction heating of the heating member by the exciting coil can be controlled.

  FIG. 1 is a cross-sectional view showing the configuration of the fixing device of the present invention. FIG. 2 is a view showing the arrangement of the coils of the fixing device of the present invention. FIG. 1 shows a cross section perpendicular to the axis of the fixing roller of the fixing device of the present invention, and is a cross sectional view of a portion where the magnetic core 5b exists near the end of the fixing roller 2 in FIG. In FIG. 2, for convenience of explanation, the magnetic cores 5 a and 5 c are omitted from the magnetic core 5, and only the magnetic cores 5 b and 5 d arranged at positions where there are no coil windings are illustrated. . Further, the fixing roller 2 is located directly below the coil and should be overlapped with the coil, but is difficult to see.

As shown in FIG. 1, a fixing device 10 according to the present invention includes a sub induction coil 1 corresponding to a demagnetizing coil disposed on a peripheral circuit of magnetic flux generated by an exciting coil 3, which will be described later, and a fixing roller 2 as a heating member. An exciting coil 3 as a magnetic field generating means, a pressure roller 4 that forms a nip region with the fixing roller 2, and a magnetic core that prevents the magnetic field generated by applying an alternating current to the exciting coil 3 from leaking outside. 5.

  In the fixing device 10 of the present invention, as shown in FIG. 2, the fixing roller 2 has an outer diameter of 40 mm, a length of 320 mm, and can heat and fix a sheet of A3 size (width: 297 mm) at the maximum. The magnetic field generated by the exciting coil 3 induction heats the heat generating layer 21 that is the surface layer of the fixing roller 2. While the recording member 6 is pressurized by passing through a nip region constituted by the fixing roller 2 and the pressure roller 4, heat is applied from the fixing roller 2 and the toner on the recording member 6 is heated and fixed to the recording member 6. The When heating and fixing a sheet having a width smaller than the width of the fixing roller 2 such as an A4 size (width: 210 mm) sheet, the center of the A4 size sheet is aligned with the center of the fixing roller 2 as shown in FIG. To use.

  Further, the exciting coil 3 is composed of a bundle of 90 copper wires having an outer diameter of 0.15 mm, whose surface is insulated. As shown in FIG. It circulates 10 times along 2. Although not shown, the exciting coil 3 is connected to a power source that supplies an alternating current for generating a magnetic field. As can be seen from FIG. 2, the exciting coil 3 is bent in a semicircular shape near the linear portion where the wire bundle is parallel and straight to the rotation axis direction of the fixing roller 2 and the end of the fixing roller 2. There is a curved part. Since the strength of the generated magnetic field is different between the linear portion and the curved portion, the length of the exciting coil 3 in the linear direction is set so that the linear portion substantially matches the length of the fixing roller 2 or slightly longer, thereby reducing the influence of the curved portion. ing. This is to make the heat generation in the direction of the rotation axis of the fixing roller 2 uniform.

  Further, the auxiliary induction coil 1 uses the same wire bundle as that used for the exciting coil 3 and is disposed so as to face the vicinity of both ends of the fixing roller 2 as shown in FIGS. In the present invention, the magnetic core 5b is disposed along the exciting coil 3 so as to surround the end portion side from the center of the fixing roller 2 along the axial direction from about 105 mm. As will be described in detail later, this is because the auxiliary induction coil 1 efficiently demagnetizes the non-sheet passing area at the end of the fixing roller 2 when fixing A4 size paper. The auxiliary induction coil 1 is rotated six times less than the exciting coil 3 along the surface of the fixing roller 2. Similar to the exciting coil 3, the auxiliary induction coil 1 is disposed at a position facing the heat generating layer 21, and is installed on the inner side of the encircling part of the exciting coil 3 and on the fixing roller 2 side to reduce the size of the entire heating device. is there. In addition to the example of the configuration shown in FIG. 1, the auxiliary induction coil 1 is provided between the exciting coil 3 and the heat generating layer 21 as shown in FIG. 3A, or as shown in FIG. 3 may be formed between the exciting coil 3 and the magnetic core 5a, but the demagnetization performance of the auxiliary induction coil is shown in FIG. Although the configuration and arrangement shown in FIG. 3A, the configuration and arrangement shown in FIG. 3B, and the configuration and arrangement shown in FIG. 1 are higher in this order, demagnetization is performed by a capacitor connected to a sub induction coil described later in this embodiment. Since the performance can be improved, any configuration may be employed as long as the auxiliary induction coil is in a position that circulates a part of the alternating magnetic flux generated by the exciting coil.

  Further, as shown in FIGS. 1 and 2, the magnetic core 5 includes a first magnetic core 5 a disposed behind the exciting coil 3 so as to face approximately a half circumferential surface of the heat generating layer 21 of the fixing roller 2, and A second magnetic core 5b extending from the first magnetic core 5a to the central portion of the auxiliary induction coil 1, a third magnetic core 5d extending to the central portion of only the exciting coil 3, and a first magnetic core The fourth magnetic core 5c is disposed so as to surround the exciting coil 3 at the end of 5a. These magnetic cores 5a, 5b, 5c, and 5d are intended to efficiently cause the magnetic flux to reach the heat generating layer 21 of the fixing roller 2, and are preferably made of a ferromagnetic material made of a material having high electrical resistivity. Typical materials include ferrite and permalloy. Although not clearly shown in FIG. 1, the magnetic core 5a may be a curved plate-like pair or a shape in which curved rod-like bodies are arranged in the form of a plurality of cores.

  FIG. 4 is a circuit diagram showing a sub induction circuit in the fixing device of the present invention. The induction circuit in the fixing device of the present invention shown in FIG. 1 includes a secondary induction coil 1, a switch 11, and electrostatic capacity switching means 12 that switches the generation direction of alternating magnetic flux of the secondary induction coil 1. In the fixing device of the present invention, a mechanical relay is used as the switch 11, but in addition to this, a triac, FET, IGBT, or the like can also be used, and a magnetoresistive effect element whose electric resistance changes according to a change in an external magnetic field. Any configuration may be used as long as the switching of the sub induction circuit can be switched, such as a configuration in which the current of the sub induction coil is changed by applying magnetism at an arbitrary timing or the magnetic field generated by the exciting coil. Capacitance switching means 12 switches to one of a plurality of capacitors having different capacitances, as described later, or changes the capacitance of the variable capacitor in order to switch the generation direction of the alternating magnetic flux of the sub induction coil 1. Thus, the magnitude of the induced current due to the electromotive force generated by the exciting coil 3, the waveform of the alternating current phase, and the resonance behavior with the alternating current of the exciting coil 3 in the auxiliary induction circuit are adjusted. As an example of the capacitance switching means 12 in the fixing device of the present invention, as shown in FIG. 5, two capacitors 12-1 and 12-2 having different predetermined capacitances, and any one of the capacitors 12- And a switch 12-3 for switching between 1 and 12-2. Further, as another example of the capacitance switching means 12, as shown in FIG. 6, it is composed of a variable capacitor 12-4, and the capacitance of the variable capacitor 12-4 is varied.

  Further, as shown in FIGS. 1 and 2, the fixing roller 2 that is a heating member in the fixing device 10 of the present embodiment has a length of 320 mm in the rotation axis direction and an outer diameter of 40 mm, and is formed on the surface of the heat generating member. The release layer, the conductive heat generating layer 21 as the heat generating member main body, the elastic layer 22, and the cored bar layer 23 are included. As shown in FIG. 1, the positional relationship of each layer is in the order of a release layer, a heat generation layer 21, an elastic layer 22, and a cored bar layer 23, and is different from a heat roller of a halogen lamp. In FIG. 1, the release layer is shown as an integral part of the heat generating layer 21.

  The heat generating layer 21 is formed of a metal member having good conductivity and good heat conductivity that is easy to generate eddy current by an alternating magnetic field and is suitable for electromagnetic induction heating. A metal having a high resistance is generally known as a metal suitable for electromagnetic induction heating. However, even if the resistance is low, a metal member having a good heat transfer property is thinned so that the heat generation layer 21 is substantially formed. The resistance value can be arbitrarily set and the calorific value can be adjusted. In the present embodiment, the heat generating layer 21 has a structure in which copper having a thickness of 10 μm is plated on a nonmagnetic stainless steel layer having a thickness of 50 μm. Since the heat generating layer 21 may be of good electrical conductivity and good heat conductivity, other metal layers such as silver, aluminum, magnesium, etc., or nickel or magnetic stainless steel, which are magnetic materials, may be used.

  A release layer is formed on the surface of the heat generating layer 21 that is the outermost layer of the fixing roller 2 so as not to adhere the toner on the recording member, although it is not clearly shown in FIG. The release layer may be a fluororesin such as PTFE, PFA, FEP, a mixture of these resins, or a dispersion of these resins in a heat resistant resin. The release layer has a thickness of 5 to 50 μm (preferably 10 to 30 μm). Thereby, the recording member passing through the fixing roller 2 and the toner releasability are improved.

  Further, the elastic layer 22 is made of an elastic material such as fluorine rubber, silicon rubber, or fluorosilicon rubber. The elastic layer 22 can increase the width of the nip region, control the paper discharge direction by adjusting the hardness, and improve the separation performance of the recording member. The elastic layer 22 is made of sponge rubber to suppress heat transfer to the inside of the fixing roller 2 so as to keep the heat of the heat generating layer 21 in a heat-insulating state, and the surface layer of the fixing roller 2 is rapidly heated to a temperature required for fixing. Even if the recording member 6 is deprived of heat, heat can be supplied smoothly. In the present embodiment, foamed silicon rubber having a thickness of 7 mm is used for the elastic layer 22.

  The cored bar layer 23 is a support for the entire fixing roller 2 and uses a metal such as iron or aluminum in order to ensure rigidity against a load for forming the nip region. It is also possible and preferable that the cored bar layer is made of a nonmagnetic or insulating material such as nonmagnetic stainless steel or ceramic so as not to affect induction heating. In this embodiment, SUS304, which is a nonmagnetic stainless steel having an outer diameter of 22 mm and a thickness of 2.0 mm, is used, and the energy of induction heating can be concentrated in the heat generating layer without loss.

  Next, the operation of the fixing device of this embodiment having such a configuration will be described below. By passing a high-frequency alternating current of about 10 kHz to 1 MHz through the exciting coil 3, the magnetic lines of force are formed alternately in the bidirectional direction in the loop of the exciting coil 3. Thereby, Joule heat is generated by the eddy current generated in the heat generating layer 21, and the surface of the heat generating layer 21 is heated. The fixing roller 2 is driven to rotate in the direction of the arrow in FIG. At the same time, the pressure roller 4 also rotates while being pressed against the fixing roller 2 at the nip portion. The recording member 6 on which the toner image is unfixed is conveyed to the nip portion, and is conveyed to the opposite side of the fixing roller 2 while being pressed against the nip portion. At this time, the heat on the surface of the heat generating layer 21 of the fixing roller 2 is applied to the toner on the recording member 6 to be heat-pressed to fix the toner image on the recording member 6.

  Since the heat generated in the heat generating layer 21 in the surface layer portion of the fixing roller 2 is heat-insulated and held by the elastic layer 22, the temperature of the surface layer portion, which is a thin layer, quickly rises, and the start-up characteristics of the fixing device are improved. Very good. The start-up characteristic means a short heating time up to a temperature required for the fixing roller 2 to fix the toner. The shorter the heating time, the easier the user to use the image forming apparatus. In this embodiment, the fixing set temperature required for the start-up is 170 ° C., and the start-up time when the heating power of 1200 W is applied is 10 seconds.

  Next, a mechanism for suppressing the excessive temperature rise of the fixing roller non-sheet passing portion by the auxiliary induction coil will be described. FIG. 7 is a diagram for explaining a heating control principle in which a portion related to induction heating in FIG. 1 is extracted. (A) of FIG. 7 represents the state of the magnetic flux A when the sub induction circuit including the sub induction coil 1 is in an open state, that is, when the switch 11 is open, by arrows. The magnetic flux A generated by the exciting coil 3 passes through the heat generation layer 21 through the magnetic core 5 and returns to the magnetic core 5 again. At this time, the magnetic flux A forms a magnetic circuit that passes through the heat generating layer 21. Therefore, an induced current flows through the heat generating layer 21 and the heat generating layer 21 generates heat due to Joule heat. At this time, since the sub induction circuit including the sub induction coil 1 is electrically open, an electromotive force is generated, but no current flows therethrough, and the magnetic flux of the exciting coil 3 is not canceled. The same heating as in the non-region is performed.

  FIG. 7B shows the state of the magnetic flux when the sub induction coil is short-circuited, that is, when the switch 11 is closed. The magnetic flux A generated by the exciting coil 3 is canceled out by the reverse magnetic flux B generated from the sub induction coil 1 and becomes a weak magnetic flux. Since most of the magnetic flux generated from the exciting coil 3 passes through the auxiliary induction coil 1, a counter electromotive force is generated in the secondary coil portion for induction and a current flows because the switch 11 is closed. Due to the magnetic flux B generated in the direction of canceling A, the magnetic flux for induction heating of the heat generating layer 21 is very weak. For this reason, a weak induction current commensurate with the magnetic flux flows at the position of the heat generation layer 21 facing the sub induction coil 1, and the heat generation due to the Joule heat of the heat generation layer 21 is small. In this case, as will be described in detail later, the heat generation due to the Joule heat of the heat generation layer 21 can be adjusted by the capacitor 12 which is an induction current adjusting device arranged in the excitation circuit including the excitation coil 3.

  When the width of the recording member to be introduced into the fixing device is A3 size, the auxiliary induction coil 1 is not operated as shown in FIG. 7A, and the end of the fixing roller 2 is sufficiently heated to generate A3 size recording member. In the case of A4 size, the auxiliary induction circuit is closed and the auxiliary induction coil 1 is operated as shown in FIG. 7B to suppress the heat generation at the end of the fixing roller 2. .

  Furthermore, it demonstrates in detail using FIG. FIG. 8A shows the magnetic flux in the cross section of the central portion of the heating roller without the auxiliary induction coil 1, and the exciting coil 3 is always operating. In FIG. 8A, seven magnetic fluxes A (solid line magnetic flux) are generated by the exciting coil 3, and an eddy current is induced in the heat generating layer 21 to generate Joule heat. On the other hand, in (b) to (d) of FIG. 8, the auxiliary induction coil 1 is operated, and three magnetic fluxes among the seven magnetic fluxes A generated by the exciting coil 3 are generated by the auxiliary induction coil 1. It is erased by three magnetic fluxes (dotted magnetic flux). For this reason, the heat generating layer 21 in this region only induces eddy currents corresponding to the four magnetic fluxes A to generate Joule heat.

Next, the capacitance of the capacitor connected to the sub induction coil in FIG. 5 will be described. The heat generation suppression effect of the sub induction coil 1 varies greatly depending on the capacitance of the capacitor connected to the sub induction coil 1 of FIG. Here, FIG. 9 shows the influence of the capacitance of the capacitor calculated by the induction heating simulation based on the calculation formula for obtaining the heat generation suppression effect η shown below on the heat generation suppression effect . 9A and 9B show the same results, but the scales of the vertical and horizontal axes are different. The horizontal axis represents the capacitance of the capacitor, and the vertical axis represents the heat generation suppression effect η (= {(heat generation suppression rate when the capacitor is connected) / (heat generation suppression rate when the capacitor is not connected)} × 100. [%]). That is, the heat generation suppression effect η represents the magnitude of the heat generation suppression rate when capacitors having various electrostatic capacities are connected, where 100 is the heat generation suppression rate of the sub induction coil 1 when no capacitor is connected. Is. Further, the heat generation suppression rate is a value represented by the following expression, and is a value indicating the ratio of the heat generation amount that is reduced when the sub induction coil 1 is changed from the open state to the closed state. It is. Qoff is the amount of heat generated when the sub induction coil 1 is open, and Qon is the amount of heat generated when the sub induction coil 1 is closed.

  As can be seen from FIG. 9, the value of the vertical axis is 100% or more, that is, the capacitance of the capacitor that improves the heat generation suppression effect of the sub induction coil 1, and the value of the vertical axis is less than 100%, There is a capacitance of the capacitor in which the heat generation suppressing effect of the induction coil 1 decreases, and a capacitance of the capacitor in which the value on the vertical axis is a negative value, that is, the auxiliary induction coil 1 promotes heat generation of the heating member.

  9A, when the capacitance of the capacitor is smaller than 1 μF, the heat generation suppression effect η is almost 0%. By connecting the capacitor 12, the sub induction coil 1 is inherently It can be seen that the heat generation suppressing effect that had been lost. Furthermore, it can be seen that when the capacitance of the capacitor is changed to be larger than 1 μF, the heat generation suppression effect η gradually becomes a negative value, and the sub induction coil 1 promotes the heat generation of the heating member. The acceleration of heat generation of the heating member by the sub induction coil 1 peaks at 4.8 μF and gradually decreases. However, when the capacitance of the capacitor is varied to 6.6 μF, the heat generation is accelerated. Further, the capacitance of the capacitor at which the heat generation suppressing effect η of the sub induction coil 1 is 100% or more is the case where the capacitance of the capacitor is 8.0 μF or more from FIG. 9B. The heat generation suppression effect peaked at 110% at 8.0 μF and the capacitance of the capacitor at about 130% at 10.0 μF, and when the capacitance of the capacitor was greatly varied, the heat generation suppression effect gradually increased to 100%. Approaching.

  As a result, by switching the capacitor connected to the sub induction coil 1 to a capacitor having an appropriate capacitance, or by variably setting the capacitance of the variable capacitor connected to the sub induction coil 1, the sub induction coil It can be seen that the heat generation suppression effect of No. 1 changes greatly, and in order to suppress the heat generation of the heating member by the sub induction coil 1, an appropriate capacitance must be set. Thus, the capacitance of the capacitor that suppresses heat generation by the auxiliary induction coil 1 is related to the inductance of the auxiliary induction coil 1. In addition, one of the causes of improving the heat generation suppression effect when a capacitor is connected to the sub induction coil 1 is that the impedance of the sub induction circuit is reduced.

  From the equivalent circuit of the sub induction circuit shown in FIGS. 4 to 6, the impedance of the sub induction circuit is expressed by the following equation.

Here, R is the DC resistance component (resistance of the conductors of the auxiliary inductive coil), L is the inductance of the secondary induction coil 1, C is the capacitance of the capacitor, f is of the alternating current frequency, X _L is the inductive reactance, X _C is It is capacitive reactance.

Therefore, | X L _-X C | By connecting the _<capacitor serving as X _L, the impedance of the secondary induction circuit becomes low, it is possible to supply a large current to the auxiliary induction coil 1. That is, the capacitance C of the capacitor is at least

Must be set to be The inductance of the auxiliary induction coil 1 of the present embodiment is 8.5 μH when measured with the exciting coil 3 electrically opened after being assembled in the fixing device, and therefore the capacitance of the capacitor is at least 2 The current flowing through the auxiliary induction coil can be increased by setting it to 38 μF or more.

  Therefore, in FIG. 7A, the capacitance C of the capacitor is

In the range smaller than 2.38 μF, since the impedance of the sub induction circuit is larger than that when no capacitor is connected, the heat generation suppression effect η is not improved.

  However, as can be seen from FIG. 9A, even if C> 2.38 μF, the heat generation suppression effect is not improved, and there is a capacitance region of the capacitor that promotes heat generation of the heating member. This is considered to be caused by the phase difference between the current of the exciting coil and the current of the auxiliary induction coil.

  FIG. 10 is a vector diagram of alternating current / alternating voltage in the exciting coil and the auxiliary induction coil. (A) of FIG. 10 is a vector diagram of alternating current / alternating voltage in the exciting coil and the auxiliary induction coil when no capacitor is connected to the auxiliary induction coil, and (b) of FIG. 10 is a capacitor in the auxiliary induction coil. It is a vector diagram of an alternating current and an alternating voltage in an exciting coil and a sub induction coil when connected.

  First, the phase of the current Imain flowing through the exciting coil is delayed by about 90 ° with respect to the voltage applied to the exciting coil, that is, the voltage Vmain of the high frequency power source. In practice, the phase delay is slightly smaller than 90 ° due to the resistance components of the coil itself and the heating member. And since the counter electromotive force generated in the auxiliary induction coil is generated in the direction to cancel the magnetic flux of the exciting coil, the voltage Vsub generated in the auxiliary induction coil is 180 degrees out of phase from Vmain, and the current Isub flowing in the auxiliary induction coil is The phase is delayed by 90 ° from Vsub and 180 ° from Imain. The capacitance C of the capacitor is

By switching the capacitor connected to the sub induction coil 1 to a capacitor having an appropriate capacitance so as to satisfy the condition, or by setting the capacitance of the variable capacitor connected to the sub induction coil 1 variably, The current flowing through the coil is larger than when no capacitor is connected. On the other hand, the capacitor also has an effect of advancing the phase of current with respect to the phase of electromotive voltage.

  Therefore, as shown in FIG. 10 (b), due to the capacitance of the capacitor connected to the sub-induction coil, the current of the sub-induction coil is not connected to the capacitor as in IsubA and IsubB. The phase advances from the current Isub.

  When a current such as IsubA flows in the current of the secondary induction coil, the direction of the current cancels Imain even if a current larger than the current Isub of the secondary induction coil without the capacitor connected flows. Since it is not suitable, heat generation is not suppressed. When a current larger than Isub flows and a current such as IsubB flows in a direction that cancels the current Imain of the exciting coil, the heat generation suppressing effect is improved for the first time.

  In FIG. 9 (a), the heat generation of the heating member is promoted from 1 μF to 6.6 μF because the direction of the current flowing in the auxiliary induction coil cancels the current flowing in the exciting coil due to the phase difference phenomenon described above. It is because it is not facing.

Further, the acceleration of heat generation of the heating member by the sub induction coil 1 has a peak at 4.8 μF because the resonance frequency wave number f ₀ calculated from the inductance L of the sub induction coil 1 and the capacitance C of the capacitor is. This is because the frequency f of the alternating magnetic field of the exciting coil is 25 kHz.

The resonance frequency f ₀ is expressed by the following equation.

Therefore, the capacitance C of the capacitor where the frequency f of the alternating magnetic field of the exciting coil = resonance number wave number f ₀ is

If the inductance L of the auxiliary induction coil is 8.5 μH and the frequency f of the alternating magnetic field of the exciting coil is f = 25 kHz, 4.77 μF is obtained.

  When the capacitance switching means of the sub-induction circuit of the present embodiment is switched to a capacitor having a capacitance of 4.77 μF, or the capacitance of the variable capacitor is variably set to 4.77 μF, the sub-induction The impedance Z of the circuit is

Becomes the smallest, and the current flowing through the auxiliary induction coil becomes the maximum. However, it can be seen from FIG. 9 that heating is promoted because the direction of the current flowing through the auxiliary induction coil is not directed to cancel the current flowing through the exciting coil due to the phenomenon of the phase difference described above.

Therefore, in order to improve the heat generation suppressing effect of the auxiliary induction coil, at least the capacitance of the capacitor connected to the auxiliary induction coil is equal to the resonance frequency f calculated by the capacitance of the capacitor and the inductance L of the auxiliary induction coil. _It is necessary to set the capacitance of the capacitor satisfying the following formula (1), where ₀ is lower than the frequency f of the alternating magnetic field of the exciting coil.

  However, f is the frequency (Hz) of the alternating magnetic field of the exciting coil, L is the inductance (H) of the auxiliary induction coil, and π is the circumference.

  More preferably

It is to set to the capacitance of the capacitor that satisfies the above.

  The capacitance reactance equation is the upper limit of the capacitance of the capacitor.

As can be seen from the above, when the capacitance of the capacitor increases, the effect on the induction circuit is reduced, and the meaning of connecting the capacitor to the auxiliary induction coil is lost.

  Therefore, in the present embodiment, as shown in FIG. 9B, when the capacitance of the capacitor exceeds 100 μF, the effect of connecting the capacitor is lost, and the heat generation suppressing effect is 100%.

  Therefore, the capacitance C of the capacitor is

It is more preferable to set the capacitance of the capacitor to satisfy

  As a result, the demagnetization performance can be improved without changing the shape of the auxiliary induction coil, the geometric shape such as the number of turns. In the present embodiment, the capacitance of the capacitor is set to 10 μF according to the above formula (2).

  Next, the control of the auxiliary induction coil in the fixing device of the present invention will be described. As described with reference to FIGS. 1 and 2, in this embodiment, a set of auxiliary induction coils is arranged at the positions of both ends of the heating roller corresponding to the non-sheet passing area when A4 size paper is passed. Yes. Since the maximum width of the recording paper that can be processed in the present embodiment is A3 size, when passing the A3 size recording paper, a control unit (not shown) that has received a signal indicating the passing of the A3 size recording paper. The secondary induction coil 1 is opened based on the control signal from the control device (1) and the entire area of the fixing roller 2 is heated. When A4 size recording paper is passed, fixing is performed from positions corresponding to both ends of A4 size based on a control signal from a control unit (not shown) that has received a signal indicating A4 size recording paper passing. The auxiliary induction coil 1 arranged over the end of the roller 2 is closed to reduce the amount of heat generated in the non-sheet passing area of the fixing roller 2. By arranging the set of sub induction coils 1 at positions opposite to the non-sheet passing areas at both ends of the fixing roller 2 when passing small size paper in this way, the sub induction circuit is opened and closed. Even when recording papers of different sizes are passed, the temperature distribution in the rotation axis direction of the fixing roller 2 can be appropriately controlled. Note that the present invention can also be applied to recording paper on which images having different image area widths are formed.

  The fixing device includes a temperature detecting means (not shown) on the surface of the fixing roller 2, and can control power application to the exciting coil 3, opening / closing of the auxiliary induction coil 1, and current amount according to the detected temperature. The temperature detection means may be a thermistor, but it is desirable to use a non-contact type such as a thermopile or an infrared temperature sensor so as not to be affected by induction heating. Further, it is preferable that the temperature detecting unit can measure a plurality of points along the rotation axis direction of the fixing roller 2. In particular, it is preferable to be able to measure the temperature at the positions where the sheet passing area and the non-sheet passing area depend on the type of recording member assumed. If the auxiliary induction current adjusting device capable of adjusting the auxiliary induction current stepwise or continuously is provided, the current value of the auxiliary induction coil 1 can be adjusted corresponding to the detected temperature, and more precise temperature adjustment is possible.

  FIG. 11 is a diagram illustrating a heat generation amount distribution of the fixing roller in the fixing device according to the first embodiment. As shown in the figure, the amount of heat generated when passing a large size (for example, A3 size) recording sheet is substantially the same from end to end with respect to the rotation axis direction of the fixing roller 2. On the other hand, when passing a small size (for example, A4 size) recording sheet, the A4 sheet passing area of the fixing roller 2 generates almost the same amount of heat as the A3 sheet passing, but both ends of the fixing roller 2 The calorific value is reduced in the non-sheet passing area.

  FIG. 12 is a diagram showing a heat generation amount distribution of the fixing roller in the fixing device of the present embodiment. The calorific value distribution shown in the figure is the distribution of the calorific value of the fixing roller in the fixing device of the present embodiment and the condenser when a small size (for example, A4 size) paper is passed through the fixing device of the present embodiment. 3 is a comparison of the calorific value distribution of the fixing roller in a fixing device including a sub induction coil not connected to the fixing roller. As the fixing device provided with the auxiliary induction coil not connected with the capacitor, the fixing device provided with the auxiliary induction coil having the same wire bundle as the auxiliary induction coil of the present embodiment and having the circulation number of 3 times, and the auxiliary device having the rotation number of 6 times. A fixing device with an induction coil was used. As can be seen from the figure, the calorific value distribution of the fixing roller of the fixing device provided with the auxiliary induction coil having the number of turns of three times without connecting the capacitor is not compared with the present embodiment. The amount of heat generated in the area is high. This indicates that the heat generation suppressing effect of the auxiliary induction coil is lower than that of the present embodiment. Although the calorific value distribution of the fixing roller of the fixing device having the auxiliary induction coil having six turns without a capacitor connected is slightly larger than that of the present embodiment, the heat generation in the non-sheet passing regions at both ends is suppressed. Since the number of turns of the wire bundle is doubled, the size of the auxiliary induction coil becomes larger than that of the present embodiment.

  FIG. 13 is a diagram showing a temperature change of the surface of the fixing roller when A4 size paper is continuously fed in the fixing device of the present embodiment. For comparison, the figure also shows the temperature change of the surface of the fixing roller of the fixing device provided with the auxiliary induction coil having three turns without a capacitor. In this embodiment, when the temperature of the non-sheet passing area of the fixing roller becomes equal to or higher than the first set temperature, the auxiliary induction coil is closed, and when the temperature is equal to or lower than the second set temperature, the auxiliary induction coil is turned off. Control to make it open. In this embodiment, the second set temperature is set to 170 ° C., which is the fixing set temperature, and the first set temperature is set to 190 ° C., which is 20 ° C. higher than the set fixing temperature. A broken line is a temperature at a substantially central portion of the fixing roller which is a sheet passing area. Control is performed so that the fixing set temperature of 170 ° C. is maintained before and after the start of paper feeding. Here, the “no auxiliary induction coil” indicated by the dotted line is the temperature at the end of the heating roller, which is a non-sheet passing region, assuming that the auxiliary induction coil is left open and there is no auxiliary induction coil. Since the sheet that absorbs heat is not conveyed, the surface temperature of the fixing roller increases with time, and in this case, the temperature becomes approximately 220 ° C. after 100 seconds from the start of sheet passing. In addition, the alternate long and short dash line “3 turns of auxiliary induction coil and no capacitor” indicates that when the temperature at the end of the fixing roller exceeds 190 ° C. after the start of paper feeding, the secondary induction coil not connected to the capacitor is closed. This is a case where the amount of heat generated at the end of the fixing roller is suppressed. In this case, the surface temperature of the end portion of the fixing roller gradually increases as compared with “no auxiliary induction coil”. This is because the effect of suppressing heat generation is not sufficient because the number of turns of the auxiliary induction coil is as small as three. The surface temperature of the fixing roller increased with time, and although not shown, the temperature exceeded approximately 200 ° C. after 1200 seconds from the start of paper feeding. Further, the solid line shown as “this embodiment” indicates that after the start of sheet passing, when the temperature of the end of the fixing roller exceeds 190 ° C., the auxiliary induction coil of this embodiment is closed and the fixing roller This is a case where the amount of heat generated at the end is suppressed. In this case, the surface temperature of the end portion of the fixing roller started to decrease immediately after the auxiliary induction coil was closed, and became a steady value and stabilized after 20 seconds.

  As described above, in the conventional fixing device in which the auxiliary induction coil does not operate, or in the auxiliary induction coil having three turns without a capacitor, the temperature of the non-passing area where the heat is not taken away by the recording paper continues to rise. The member may be damaged. In the fixing device of the present invention, when the end temperature reaches the second set temperature of 190 ° C., the auxiliary induction coil is closed to sufficiently suppress heat generation, so that the temperature of the fixing roller is kept uniform. You can see that In an actual image forming apparatus, if the temperature of the non-sheet passing region is too low and falls below 170 ° C., it is preferable to keep the auxiliary induction coil open or above 170 ° C. In this way, there is almost no influence on the paper passing area. It is also possible to switch to A3 size paper at any time. As described above, the sub induction coil is switched to a capacitor having a capacitance satisfying the above formula (2), or the capacitance is changed to a capacitance satisfying the above formula (2), more preferably from the above formula (1). By variably setting the capacitance of the variable capacitor, a magnetic core is installed in the magnetic flux path of the exciting coil and the auxiliary induction coil to increase the coupling, or the number of turns of the auxiliary induction coil is increased to increase the shape. Therefore, it is possible to provide a fixing device that can improve the heat generation suppressing effect of the auxiliary induction coil and can perform a good fixing operation even if the recording members having different sizes are continuously passed through.

FIG. 14 is a sectional view showing the coil arrangement of the fixing device according to the second embodiment of the present invention. 2 is a cross-sectional view of a portion where the magnetic core 5b is present near the end of the fixing roller 2 in FIG. In the figure, the same reference numerals as those in FIG. 2 denote the same components. This embodiment is different from the first embodiment only in the arrangement of the auxiliary induction coil in the direction of the rotation axis of the fixing roller and the capacitance of the capacitor connected to the auxiliary induction coil. It is the same as the embodiment and the fixing device. Therefore, the cross-sectional view of the fixing device of the present invention in a cross section perpendicular to the axis of the fixing roller is the same as FIG. As shown in the figure, the auxiliary induction coil 1 is disposed so as to face the vicinity of the center portion of the fixing roller 2. In the present embodiment, the magnetic core 5d is disposed along the exciting coil 3 so as to surround a range of 105 mm on both sides along the axial direction from the center of the fixing roller 2. As will be described in detail later, this is because the auxiliary induction coil 1 efficiently promotes heating of the sheet passing area of the fixing roller 2 when fixing A4 size paper. Further, the capacitance of the capacitor connected to the auxiliary induction coil 1 is set to an electrostatic capacitance that promotes heating in a range where the auxiliary induction coil 1 of the fixing roller 2 faces by closing the auxiliary induction coil 1. It is set. Specifically, as described in FIG. 9A, in this embodiment, the resonance frequency wave number f ₀ calculated from the inductance L of the auxiliary induction coil and the capacitance C of the capacitor is the alternating number of excitation coils. When the frequency f of the magnetic field is 25 kHz, the heating of the heating member by the auxiliary induction coil 1 is peaked, so the capacitance C of the capacitor is

A capacitance in the vicinity of is preferable. Therefore, if the inductance L = 10 μH of the sub induction coil of this embodiment and the frequency f = 25 kHz of the alternating magnetic field of the exciting coil are substituted, a capacitance in the vicinity of 4.05 μF is preferable. The nearby electrostatic capacity indicates the electrostatic capacity of the capacitor set so as to satisfy the following expression (3) from FIG.

  However, f is the frequency (Hz) of the alternating magnetic field of the exciting coil, L is the inductance (H) of the auxiliary induction coil, and π is the circumference. In this embodiment, the capacitance of the capacitor is 3 μF.

Next, a mechanism for promoting the temperature rise of the sheet passing portion of the fixing roller by the auxiliary induction coil will be described.
FIG. 15 is an explanatory diagram of the heating promotion principle in the fixing device according to the second embodiment. In the figure, the same reference numerals as those in FIGS. 1 and 4 denote the same components. (A) of FIG. 15 represents the state of the magnetic flux A when the auxiliary induction circuit including the auxiliary induction coil 1 is in an open state, that is, when the switch 11 is open, by arrows. The magnetic flux A generated by the exciting coil 3 passes through the heat generation layer 21 through the magnetic core 5 and returns to the magnetic core 5 again. At this time, the magnetic flux A forms a magnetic circuit that passes through the heat generating layer 21. Therefore, an induced current flows through the heat generating layer 21 and the heat generating layer 21 generates heat due to Joule heat. At this time, since the auxiliary induction circuit including the auxiliary induction coil 1 is electrically open, an electromotive force is generated, but no current flows thereby, and the magnetic flux of the exciting coil 3 is not affected. The same heating as in the non-region is performed.

  FIG. 15B shows the state of the magnetic flux when the auxiliary induction coil is short-circuited, that is, when the switch 11 is closed. The magnetic flux A generated by the exciting coil 3 overlaps with the magnetic flux B generated in the same direction from the auxiliary induction coil 1 and becomes a stronger magnetic flux. Since most of the magnetic flux generated from the exciting coil 3 passes through the auxiliary induction coil, a counter electromotive force is generated in the auxiliary induction coil and a current flows because the switch 11 is closed. At that time, due to the influence of the capacitor connected to the sub induction coil 1, the magnetic flux B originally generated in the direction of canceling out the magnetic flux A is generated in the same direction as the magnetic flux A of the exciting coil 3. The magnetic flux is very strong. For this reason, a strong induction current commensurate with the magnetic flux flows at the position of the heat generation layer 21 facing the sub induction coil 1, and the heat generation due to the Joule heat of the heat generation layer 21 becomes large.

  Next, control of the sub induction coil in the heating device of the present invention will be described. As described with reference to FIG. 14, in this embodiment, a set of auxiliary induction coils is arranged at the position of the fixing roller corresponding to the sheet passing area when A4 size paper is passed. Since the maximum width of the recording paper that can be processed in the present embodiment is A3 size, when passing the A3 size recording paper, a control unit (not shown) that has received a signal indicating the passing of the A3 size recording paper. The auxiliary induction coil is opened based on the control signal from (1), and the entire area of the fixing roller 2 is heated. On the other hand, when A4 size recording paper is passed, it corresponds to A4 size paper based on a control signal from a control unit (not shown) that has received a signal indicating A4 size recording paper passing. The auxiliary induction coil 1 disposed at the position is closed, and the amount of heat generated in the A4 sheet passing area of the fixing roller 2 is increased.

  Thus, even when a recording paper of a different size is passed by disposing the auxiliary induction coil 1 at the position facing the small size paper passing area of the fixing roller 2 and performing the opening and closing operation of the auxiliary induction circuit. The temperature distribution in the rotation axis direction of the fixing roller can be appropriately controlled. Note that the present invention can also be applied to recording paper on which images having different image area widths are formed.

  FIG. 16 is a diagram showing a heat generation amount distribution of the fixing roller in the fixing device of the present embodiment. As can be seen from the figure, the amount of heat generated when passing a large size (for example, A3 size) recording sheet is substantially the same from end to end with respect to the rotation axis direction of the fixing roller. On the other hand, when a small size (for example, A4 size) recording paper is passed, the A4 size paper passing area of the fixing roller generates almost the same amount of heat as the A3 size paper passing, but the fixing roller The calorific value has fallen at both ends. This is due to the heat generation promoting effect of the auxiliary induction coil.

  More specifically, the amount of heat generated when passing a large size (for example, A3 size) recording sheet, that is, the amount of heat generated when the auxiliary induction coil is open, is from end to end with respect to the rotation axis direction of the fixing roller. Designed to be uniform. When the auxiliary induction coil is closed, the heat generation amount of the fixing roller at the position where the auxiliary induction coil is disposed increases. Therefore, by controlling the amount of heat generated in a region through which a small size (for example, A4 size) recording paper is passed to be equal to the amount of heat generated when a large size (for example, A3 size) recording paper is passed through, it is inevitable. In particular, the heat generation amount of the fixing roller at the position where the sub induction coil is not disposed is smaller than the heat generation amount when passing a large size (for example, A3 size) recording paper.

  FIG. 17 is a diagram showing a heat generation amount distribution of the fixing roller in the fixing device of the present embodiment. The calorific value distribution shown in the figure shows the temperature change of the surface of the heating roller when A4 size paper is continuously passed through the fixing device of the present embodiment. In the present embodiment, when the temperature of the non-sheet passing area of the fixing roller becomes equal to or higher than the first set temperature, the auxiliary induction coil is closed, and when the temperature is equal to or lower than the second set temperature, the auxiliary induction coil is opened. Control is done. In this embodiment, the second set temperature is set to 170 ° C., which is the fixing set temperature, and the first set temperature is set to 190 ° C., which is 20 ° C. higher than the set fixing temperature. A broken line is a temperature at a substantially central portion of the fixing roller which is a sheet passing area. Control is performed so that the fixing set temperature of 170 ° C. is maintained before and after the start of paper feeding. Here, the “no auxiliary induction coil” indicated by the dotted line is the temperature at the end of the fixing roller, which is a non-sheet-passing region, assuming that the auxiliary induction coil is left open and there is no auxiliary induction coil. Since the heat-absorbing paper is not conveyed, the temperature of the surface of the fixing roller increases with time, and in this case, the temperature becomes approximately 220 ° C. 100 seconds after the start of paper feeding. Also, the solid line shown as “this embodiment” indicates that after the start of paper feeding, when the temperature at the end of the fixing roller exceeds 190 ° C., the auxiliary induction coil of this embodiment is closed and the fixing roller This is a case where the heat generation amount in the paper passing area is increased. In this case, since the amount of heat generated in the sheet passing area is increased as compared with the non-sheet passing area, the sheet passing area is set to a fixing set temperature of 170 ° C. with less input power than when the sub induction coil is open. Can be maintained. Therefore, the surface temperature of the end portion of the fixing roller starts to decrease immediately after the sub induction coil is closed, and becomes a steady value and stabilizes after 20 seconds. In the fixing device of the present invention, it is understood that the temperature of the fixing roller is kept uniform by closing the auxiliary induction coil when the end temperature reaches 190 ° C. which is the second set temperature.

  FIG. 18 is a cross-sectional view showing the coil arrangement of the fixing device according to the third embodiment of the present invention. 2 is a cross-sectional view of a portion where the magnetic core 5b is present near the end of the fixing roller 2 in FIG. In the figure, the same reference numerals as those in FIGS. 2 and 14 denote the same components. As shown in the figure, the auxiliary induction coil 1-1 is arranged near the center of the fixing roller 2, and the auxiliary induction coils 1-2 and 1-3 are arranged on the peripheral circuit of the magnetic flux generated by the exciting coil 3, respectively. ing. In the present embodiment, the magnetic core 5d is disposed along the exciting coil 3 so as to surround a range of 105 mm on both sides along the axial direction from the center of the fixing roller 2. When fixing A4 size paper, the sub induction coil 1-1 is a heating induction sub induction coil for efficiently promoting heating of the sheet passing area of the fixing roller 2. Here, the capacitance of the capacitor connected to the auxiliary induction coil 1-1 is such that heating of the range where the auxiliary induction coil 1 of the fixing roller 2 faces is promoted by closing the auxiliary induction coil 1-1. The capacitance is set to Further, the auxiliary induction coils 1-2 and 1-3 are used for both heating promotion / suppression combined auxiliary induction for promoting or suppressing heating in a range where the auxiliary induction coils 1-2 and 1-3 of the fixing roller 2 face each other. It is a coil. Here, the capacitance of the capacitor connected to the auxiliary induction coils 1-2 and 1-3 is determined by closing the auxiliary induction coils 1-2 and 1-3, respectively, so that the auxiliary induction coil 1 of the fixing roller 2 is closed. The capacitance in which the heating in the range where −2 and 1-3 are opposed to each other is promoted, or the capacitance is suppressed. Therefore, when fixing a small size (for example, A4 size) recording sheet, the auxiliary induction coil 1-1 for heating promotion is closed, and the auxiliary induction coils 1-2 and 1-3 of the fixing roller 2 are closed. The auxiliary induction coils 1-2 and 1-3 that are also used for heating promotion / suppression that have been changed to have a capacitance that suppresses heating in the opposing range are closed. Further, when fixing a large size (for example, A3 size) recording sheet, the auxiliary induction coil 1-1 for promoting heating is closed and the auxiliary induction coils 1-2 and 1-3 of the fixing roller 2 are closed. The auxiliary induction coils 1-2 and 1-3 for both heating promotion and suppression, which are varied to have an electrostatic capacity that promotes heating in the opposite range, are closed. Thus, by using the auxiliary induction coil for heating promotion and the auxiliary induction coil for heating promotion / suppression as either for heating promotion or for heating suppression, an appropriate fixing temperature corresponding to the lateral width of the recording member can be obtained. Control can be performed. Note that the present invention can also be applied according to the material of the recording member and the image forming area width on the recording member.

  FIG. 19 is a cross-sectional view showing a configuration of an image forming apparatus according to another invention. In the figure, the same reference numerals as those in FIG. 1 denote the same components. The entire image forming apparatus of the present invention is composed of an upper part and a lower part. A document reading device (not shown) is provided at the upper part, an image forming unit is provided below the document reading apparatus, and a sheet feeding unit 40 on which the recording member 6 is placed is provided at the lower part. The image forming unit includes a drum-shaped photoreceptor 41 that is an example of an image carrier. Around this photosensitive member 41, in the order of the arrows, a charging device 42, a mirror 43 of an exposure means, a developing device 44, a transfer device 45 for transferring a visible image onto a transfer sheet as a recording member 6 in a transfer portion, a photosensitive device A cleaning device 46 or the like having a blade 46a that is in sliding contact with the peripheral surface of the body 41 is disposed. An exposure Lb is scanned on the photoconductor 41 via a mirror 43 between the charging device 42 and the developing device 44. A registration roller 47 is provided on the upstream side of the transfer path of the transfer device 45. The recording member 6 stored in the paper feed tray of the paper feed unit 40 is sent out toward the registration roller 47 while being guided by the conveyance guide. The fixing device 10 of the present invention is disposed downstream of the transfer device 45. The recording member 6 on which the toner image is fixed by the fixing device 10 is discharged to a paper discharge tray.

  In this image forming apparatus, image formation is performed as follows. The photoconductor 41 starts to rotate, and during this rotation, the photoconductor 41 is uniformly charged by the charging device 42 in the dark, and the exposure Lb is irradiated and scanned on the exposure unit to form a latent image corresponding to the image to be created. The The latent image is moved to the developing device 44 by the rotation of the photosensitive member 41, where it is visualized by toner to form a toner image. On the other hand, the recording member 6 on the paper feed tray is fed to the position of the registration roller 47 via the conveyance path indicated by the broken line, and stops temporarily, and the toner image on the photoconductor 41 and the image position coincide with each other at the transfer portion. Wait for the timing of sending out. Thereafter, the recording member 6 is sent out from the registration roller 47 in synchronization with the movement of the photosensitive member 41 and is conveyed toward the transfer portion. The toner image on the photoreceptor 41 is transferred onto the recording member 6 by the electric field generated by the transfer device 45 at the transfer portion. The recording member 6 to which the toner image has been transferred is sent toward the fixing device 10, and the toner image is fixed while the toner image passes through the fixing device 10, and is discharged to the paper discharge unit. Further, in this image forming apparatus, the recording member 6 discharged to the automatic duplex device 48 is switched back by the automatic duplex device 48 and is conveyed to the conveyance path before the registration roller 47, so that both surfaces of the recording member 6 are detected. An image can be formed. On the other hand, the residual toner remaining on the photosensitive member 41 without being transferred by the transfer unit reaches the cleaning device 46 as the photosensitive member 41 rotates, and is cleaned while passing through the cleaning device 46 to be ready for the next image formation.

  In addition, this invention is not limited to the said embodiment, It cannot be overemphasized that various deformation | transformation and substitution are possible if it is description in a claim.

1 is a cross-sectional view illustrating a configuration of a fixing device according to a first embodiment of the present invention. It is a figure which shows arrangement | positioning of the coil of the fixing device of 1st Embodiment. It is sectional drawing which shows another example of arrangement | positioning of the sub induction coil in the fixing device of 1st Embodiment. It is a circuit diagram which shows the circuit for induction | guidance which opens and closes a sub induction coil. It is a circuit diagram which shows an example of a structure of the electrostatic capacitance switching means of FIG. It is a circuit diagram which shows the other example of a structure of the electrostatic capacitance switching means of FIG. It is a figure explaining the heating adjustment principle which extracted the part regarding the induction heating of FIG. It is sectional drawing which shows the magnetic flux in the heating roller center part of each coil of 1st Embodiment. It is a characteristic view which shows the relationship between the capacity | capacitance of the capacitor | condenser calculated by the induction heating simulation, and the heat_generation | fever suppression effect. It is a vector diagram of an alternating current and an alternating voltage in an exciting coil and a sub induction coil. FIG. 3 is a diagram illustrating a heat generation amount distribution of a fixing roller in the fixing device according to the first embodiment. FIG. 3 is a diagram illustrating a heat generation amount distribution of a fixing roller in the fixing device according to the first embodiment. FIG. 5 is a diagram illustrating a temperature change of the surface of the fixing roller when A4 paper is continuously passed through the fixing device according to the first embodiment. It is sectional drawing which shows arrangement | positioning of the coil of the fixing device which concerns on the 2nd Embodiment of this invention. It is a figure explaining the heating adjustment principle which extracted the part regarding the heating promotion of FIG. FIG. 7 is a diagram illustrating a heat generation amount distribution of a fixing roller in a fixing device according to a second embodiment. FIG. 7 is a diagram illustrating a heat generation amount distribution of a fixing roller in a fixing device according to a second embodiment. It is sectional drawing which shows arrangement | positioning of the coil of the fixing device which concerns on the 3rd Embodiment of this invention. It is sectional drawing which shows the structure of the image forming apparatus of another invention.

Explanation of symbols

1, 1-1, 1-2, 1-3; auxiliary induction coil, 2; fixing roller,
3; excitation coil, 4; pressure roller, 5; magnetic core,
6; recording member, 10; fixing device, 11, 12-3; switch,
12; Capacitance switching means, 12-1, 12-2; Capacitor,
12-4; variable capacitor, 21; heating layer, 22; elastic layer,
23: Core metal layer.

Claims (11)

In a fixing device that heats and fixes an image on a recording member using a heating member while nipping and conveying the recording member in the image forming apparatus,
An exciting coil arranged along the heating member to generate an alternating magnetic flux and electromagnetically heat the heating member;
A degaussing coil that circulates a part of the alternating magnetic flux generated by the exciting coil and generates the alternating magnetic flux;
Capacitance switching means connected to the demagnetizing coil and switching to any of a plurality of capacitors having different capacitances;
The capacitance C of the capacitor is changed by the capacitance switching means.

The fixing device is characterized by switching to the capacitor satisfying (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circumference). The capacitance C of the capacitor is changed by the capacitance switching means.

The fixing device according to claim 1, wherein the capacitor is switched to satisfy the condition (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). The capacitance C of the capacitor is changed by the capacitance switching means.

The fixing device according to claim 1, wherein the capacitor is switched to satisfy the condition (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio). The capacitance C of the capacitor is changed by the capacitance switching means.

The fixing device according to claim 1, wherein the capacitor is switched to satisfy the condition (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circular ratio).   5. The capacitor according to claim 1, wherein the capacitor is switched to one of the capacitors by the capacitance switching unit according to a material of the recording member, a lateral width of the recording member, or an image area width on the recording member. The fixing device according to claim 1. In a fixing device that heats and fixes an image on a recording member using a heating member while nipping and conveying the recording member in the image forming apparatus,
An exciting coil arranged along the heating member to generate an alternating magnetic flux and electromagnetically heat the heating member;
A degaussing coil that circulates a part of the alternating magnetic flux generated by the exciting coil and generates the alternating magnetic flux;
A capacitance varying means for varying the capacitance of the variable capacitor connected to the demagnetizing coil;
The capacitance C of the variable capacitor is changed by the capacitance changing means.

The fixing device is variable so as to satisfy (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circumference). The capacitance C of the variable capacitor is changed by the capacitance changing means.

The fixing device according to claim 6, wherein f is varied so as to satisfy (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circumference). The capacitance C of the variable capacitor is changed by the capacitance changing means.

The fixing device according to claim 6, wherein f is varied so as to satisfy (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circumference). The capacitance C of the variable capacitor is changed by the capacitance changing means.

The fixing device according to claim 6, wherein f is varied so as to satisfy (where f is the frequency of the alternating magnetic field of the exciting coil, L is the inductance of the degaussing coil, and π is the circumference).   7. The capacitance of the variable capacitor is varied by the capacitance varying means according to a material of the recording member, a lateral width of the recording member, or an image area width on the recording member. 10. The fixing device according to any one of 9 above.   An image forming apparatus comprising the fixing device according to claim 1 and forming an image on a recording member.

Images