Patent Publication Number: US-8983348-B2

Title: Fixing device and image forming apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority pursuant to 35 U.S.C. §119 from Japanese patent application numbers 2013-009109 and 2013-014194, filed on Jan. 22, 2013 and Jan. 29, 2013, respectively, the entire disclosures of which are incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a fixing device, and in particular to the fixing device employing an electromagnetic induction heating method to fix an unfixed image on a recording medium, and an image forming apparatus including such a fixing device. 
     2. Related Art 
     Conventionally, an image forming apparatus such as a copier, a printer, and the like, employs a fixing device employing electromagnetic induction, which is both fast and energy-efficient. 
     For example, JP-2006-350054-A discloses a fixing device employing the electromagnetic induction heating method, which includes a support roller as a heat roller to generate heat, a fixing support roller as a fixing roller, a fixing belt stretching around the support roller and the fixing support roller, an induction heater disposed opposite the support roller via the fixing belt, and a pressure roller pressing against the fixing support roller via the fixing belt. The induction heater is formed of a coil such as an excitation coil wound in the longitudinal direction and a core disposed opposite the coil. The fixing belt is configured to be heated at a portion opposite the induction heater. The thus-heated fixing belt heats to fix a toner image formed on a recording medium conveyed to a position opposite the fixing support roller and the pressure roller. 
     More specifically, when a high-frequency alternating current is supplied to the coil, an alternate magnetic field is formed around the coil, and an eddy current is generated near the surface of the support roller. When the eddy current is generated to the support roller as a heat roller, joule heat is generated due to electric resistance of the support roller itself. With its joule heat, the fixing belt wound around the support roller is heated. The fixing device employing the electromagnetic induction heating method as described above is known as a device with high-thermal conversion efficiency and less energy consumption, if compared to a conventional halogen heater, capable of increasing a surface temperature of the fixing belt up to a prescribed level in a short time because a heat generator used in the electromagnetic induction fixing device is directly heated by the method. 
     However, as to the fixing device employing the induction heating method, because the high-frequency alternating current is supplied to the coil, the temperature of the excitation coil rises from joule heat generated due to electrical resistance of the excitation coil. As a result, there is a concern that UL Standards for Safety are not observed due to the increase in the temperature of the coil or the excitation coil is broken. 
     As an approach to solve the above problem, a technology is known in which the excitation coil is cooled using a cooling fan. For example, JP-2000-105516-A discloses a fixing device including an insulation member between the excitation coil and the fixing roller so that the cooling efficiency of the cooling fan is improved. However, in the fixing device employing the induction heating method the coil temperature has a tendency to rise more quickly than in the conventional device when, for example, used in a high-speed printer capable of printing speeds of more than 60 cpm (copies per minute) because of an increase in the continued printing time and a high power input for the elevated temperature for short-time start-up. Currently, there is no other way to increase the power of the cooling fan in order to increase cooling efficiency, which causes problems of cost increase, increased energy consumption, increased noise, and the like. 
     SUMMARY 
     The present invention provides an improved fixing device that includes a rotary fixing member; a pressure roller pressed against the fixing member to form a nip in association with the fixing member; and an induction heating unit, as a heat source, to heat the fixing member. The induction heating unit further includes an excitation coil; a coil retainer to hold the excitation coil; and a cover member disposed opposite the coil retainer in an opposite side of the fixing member. In such an induction heating unit, a distance between the coil retainer and the cover member is narrower in a central predetermined range of the excitation coil in an axial direction of the fixing member than other portions. 
     These and other objects, features, and advantages will become apparent upon consideration of the following description of the preferred embodiments when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic structure of an image forming apparatus including a fixing device according to a first embodiment; 
         FIG. 2  illustrates a cross-sectional view of the fixing device according to the first embodiment; 
         FIG. 3  is a cross-sectional view illustrating a structure of a fixing belt; 
         FIGS. 4A and 4B  are views each illustrating a structure of an induction heating unit; 
         FIG. 5  illustrates an induction heating unit including an excitation coil and core members; 
         FIG. 6  is a top view illustrating a state in which the excitation coil is fixed on a coil retainer; 
         FIG. 7  illustrates a cross-sectional view of the induction heating unit seen from a longitudinal direction thereof. 
         FIG. 8  is a cross-sectional view illustrating a structure of the induction heating unit according to a first comparative example; 
         FIG. 9  is a cross-sectional view illustrating a structure of the induction heating unit according to a second comparative example; 
         FIG. 10  is a graph illustrating a temperature of the coil when heated in a first embodiment and a first comparative example; 
         FIG. 11  illustrates a cross-sectional view of the induction heating unit seen from a longitudinal direction thereof according to a second embodiment; 
         FIG. 12  illustrates a cross-sectional view of the induction heating unit seen from the longitudinal direction thereof according to a third embodiment; 
         FIG. 13  illustrates a cross-sectional view of the induction heating unit seen from the longitudinal direction thereof according to a fourth embodiment; 
         FIG. 14  illustrates a cross-sectional view of a fixing device employing a heat roll method according to a fifth embodiment; 
         FIG. 15  is a cross-sectional view of the induction heating unit configured alternatively according to a sixth embodiment; 
         FIG. 16  illustrates a cross-sectional view of the fixing device according to a seventh embodiment; 
         FIG. 17  illustrates a cross-sectional view of an induction heating unit according to the seventh embodiment; 
         FIG. 18  schematically illustrates a structure of an excitation coil according to the seventh embodiment; 
         FIG. 19  is an oblique view illustrating a specific configuration of a portion at which the excitation coil is disposed according to the seventh embodiment; 
         FIG. 20  is an oblique view illustrating a rectifying member disposed on the inner side of the excitation coil according to the seventh embodiment; 
         FIG. 21  illustrates a cross-sectional view of the induction heating unit at a center portion in the longitudinal direction thereof; 
         FIG. 22  schematically illustrates a function of the rectifying member according to the seventh embodiment; 
         FIG. 23  schematically illustrates a conventional structure of the induction heating unit without a rectifying member; 
         FIGS. 24A and 24B  schematically illustrate a bent portion of the excitation coil according to a present embodiment and a comparative example; 
         FIG. 25  is a graph illustrating a result of an experiment comparing the present embodiment and the comparative example; 
         FIG. 26  schematically illustrates a conventional structure of the induction heating unit with a rectifying member; 
         FIG. 27  illustrates a rectifying member having a square leading end protruding toward an upstream side; 
         FIGS. 28A and 28B  schematically illustrate a target in designing and an actually molded product in a background art; 
         FIGS. 29A and 29B  schematically illustrate a target in designing and an actually molded product in the present embodiment; and 
         FIG. 30  illustrates an eighth embodiment in which the seventh embodiment is applied to a fixing device employing a heat roll. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred embodiments will be described with reference to accompanying drawings. 
       FIG. 1  is a schematic cross-sectional view of an image forming apparatus including a fixing device according to embodiments. Hereinafter, a structure and operation of the image forming apparatus will be described with reference to  FIG. 1 . 
     The image forming apparatus is a printer that employs an electrophotographic method and includes four sets of image forming units  10 Y,  10 M,  10 C, and  10 Bk, each mainly including photoreceptor drums  1 Y,  1 M,  1 C, and  1 Bk as an image carrier, so that a full-color image using four colors of toner, yellow (Y), magenta (M), cyan (C), and black (Bk) can be formed. However, the structure of the image forming apparatus is not limited to the illustrated example alone. For example, the illustrated printer herein employs a direct transfer method, in which a toner image is directly transferred onto a recording medium such as a sheet; however, the printer may employ an indirect transfer method, in which the toner image is transferred to the sheet via an intermediate transfer member. In addition, the number or order of colors can be varied. Further, the present invention is not limited to a printer but is applicable to a copier, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices. 
     As illustrated in  FIG. 1 , the four sets of image forming units  10 Y,  10 M,  10 C, and  10 Bk are disposed in parallel along an upper surface of a conveyance belt  20 , to thus form a tandem-type image forming section. The conveyance belt  20  is stretched around a driving roller  26  and a driven roller  27  and rotates in the direction of the arrow in the figure. A paper tray  21  to contain a sheet P as a recording medium is disposed beneath the conveyance belt  20 . The sheet P fed from a sheet feed roller pair  22  is conveyed by a conveyance roller pairs  23 ,  24  guided by a guide member, not shown, and is conveyed. The thus-conveyed sheet P is then fed to an upper surface of the conveyance belt  20  through an inlet portion where a bias roller  25  is disposed opposite the driven roller  27  and is conveyed by being electrostatically attracted to the conveyance belt  20 . Then, toner images are sequentially transferred from the image forming units  10 Y,  10 M,  10 C, and  10 Bk in the tandem image forming section to the sheet P that is conveyed by the conveyance belt  20 . The sheet carrying an unfixed toner image thereon is conveyed from the conveyance belt  20  to a fixing device  40 , and the fixing device  40  fixes the toner image onto the sheet with heat and pressure. 
     The four sets of image forming units  10 Y,  10 M,  10 C, and  10 Bk each are similar in the structure. Therefore, the extreme upstream image forming unit  10 Y is taken as representative and is described in detail below. For simplicity, reference numerals for the image forming units  10 M,  10 C, and  10 Bk other than the yellow image forming unit  10 Y are omitted. In addition, suffixes representing different colors Y, M, and C will also be omitted in the explanation below. 
     Each image forming unit  10  includes a central photoreceptor drum  1  rotatably contacting the conveyance belt  20 . Around a circumference of the photoreceptor drum  1  are disposed a charger  2 , an exposure device  3 , a developing device  4 , a transfer roller  5 , a cleaner  6 , and a discharger, not shown, in this order along a rotation direction of the photoreceptor drum  1 . The charger  2  charges a surface of the photoreceptor drum  1  to have a predetermined electric potential. The exposure device  3  exposes the charged drum surface based on color-decomposed image signals and forms an electrostatic latent image on the surface of the drum. The developing device  4  supplies toner to develop the electrostatic latent image formed on the drum surface and renders the latent image visible. The transfer roller  5  transfers the developed toner image on the sheet conveyed via the conveyance belt  20 . The cleaner  6  removes residual toner remaining, without being used in the transfer, on the surface of the drum. The discharger, not shown, removes any electrical charge remaining on the surface of the drum. 
     Next, the fixing device according to an embodiment will be described with reference to  FIG. 2 . 
       FIG. 2  is a schematic cross-sectional view of a fixing device employing induction heating method, which can be used as the fixing device  40  in the printer schematically illustrated in  FIG. 1 . As illustrated in  FIG. 2 , the fixing device  40  includes a heat roller  41 , a fixing roller  42 , a fixing belt  43 , a pressure roller  44 , and an induction heating unit  50 . 
     The heat roller  41  includes a metal core formed of non-magnetic stainless steel, having a thickness of from 0.2 to 1.0 mm. The heat roller  41  includes a heat-generating layer formed of Cu having a thickness of from 3 to 15 μm on the surface thereof, to thus improve the heat-generating effect. In this case, nickel coating is preferably applied on the surface of the Cu layer for preventing corrosion. Alternatively, any magnetic shunt alloy with a Curie point of approximately 160 to 220 degrees C. may be used. In this case, the magnetic shunt alloy can be used as a heat-generating layer, or alternatively, a Cu layer of from 3 to 15 μm may be formed, as a heat-generating layer, on the surface of the magnetic shunt alloy. Because the magnetic shunt alloy includes aluminum, temperature increase stops around the Curie point. 
     The fixing roller  42  includes a metal core  42   a  formed of, for example, stainless steel, carbon steel, or the like, and an elastic material  42   b  covering the metal core with solid or foamed silicon rubber having heat resistivity. Then, the pressure roller  44  presses against the fixing roller  42 , so that a contact portion, that is, a fixing nip N, having a predetermined width, is formed between the pressure roller  44  and the fixing roller  42 . An external diameter of the fixing roller  42  is from 30 to 40 mm, the thickness of the elastic material  42   b  is from 3 to 10 mm, and the roller hardness is from 10 to 50 degrees according to Japanese Industrial Standards Class A (JTS-A). 
     The fixing belt  43  serving as a fixing member is stretched around the heat roller  41  and the fixing roller  42 . The fixing belt  43  according to the present embodiment as illustrated in  FIG. 3  includes a base  43   a , an elastic layer  43   b , and a release layer  43   c . The elastic layer  43   b  and the release layer  43   c  are laminated on the base  43   a.    
     Properties required for the base  43   a  include mechanical strength required when stretched around the rollers, flexibility, and heat resistivity capable of withstanding the fixing temperature. In the present embodiment, the base  43   a  to induction-heat the heat roller  41  is preferably formed of insulating heat-resistant resins, such as polyimide, polyamideimide, polyetheretherketone (PEEK), polyethersulfone (PES), polyphenylene sulfide (PPS), fluorine resins, and the like. The thickness thereof is from 30 to 200 μm for thermal capacity and strength. 
     The elastic layer  43   b  is provided to give flexibility to the surface of the belt so as to obtain a uniform image without uneven glossiness, and preferably has a rubber stiffness of from 5 to 50 degrees (according to JIS-A), and a thickness ranging from 50 to 500 μm. In addition, preferable materials include silicon rubbers, fluorosilicon rubbers, and the like, for obtaining heat resistivity for the fixing temperature. 
     Materials used for the release layer  43   c  include fluorin resins such as: polytetrafluoroethylene (PTFE); tetrafluoroethylene-perfluoroalkyl vinylether copolymer (PFA); and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or mixture of these resins, or heat-resistant resins dispersed with above resins. 
     When the elastic layer  43   b  is coated with the release layer  43   c , toner can be released easily and paper dust can be prevented from sticking without using silicon oil and the like, and an oil-less structure is enabled. However, resins having good releaseability usually have no elasticity like a rubber material, so that if the thick release layer is formed on the elastic layer, elasticity of the belt surface forming the release layer is lost, thereby generating uneven glossiness in the output image. To strike the optimum balance between releaseability and elasticity, the thickness of the release layer  43   c  preferably ranges from 5 to 50 μm and is more preferably from 10 to 30 μm. 
     In addition, if necessary, a primer or undercoat layer is disposed between adjacent layers. Further, a layer to improve durability against slidable movement can be disposed on an interior surface of the base  43   a.    
     The base  43   a  may include a heat-generating layer. For example, a layer in which a Cu layer having a thickness of 3 to 15 μm is formed on the base layer formed of polyimide can be used as a heat-generating layer. 
     The pressure roller  44  is formed of a release layer  44   c , an elastic layer  44   b  having a high heat resistance, and a metal core  44   a  including a metallic cylinder portion. The pressure roller  44  presses against the fixing roller  42  via the fixing belt  43 , so that a fixing nip N is formed at the pressed portion. An outer diameter of the pressure roller is set to approximately 30 to 40 mm and the elastic layer  44   b  has a layer thickness of 0.3 to 5 mm and has an Asker stiffness of 20 to 50 degrees. A favorable material for the pressure roller  44  is silicon rubber due to the need for heat resistance. Further, in order to improve releaseability when duplex printing is performed, the release layer  44   b  formed of fluorine resins and having a layer thickness of 10 to 100 μm is disposed on the elastic layer  44   b.    
     Because the stiffness of the pressure roller  44  is greater than that of the fixing roller  42 , so that the pressure roller  44  bites into the fixing roller  42  and the fixing belt  43 . As a result, the recording medium that is conveyed along the fixing belt  43  is distorted on the way out of the fixing nip and has a curvature relative to the surface of the fixing belt  43 , and thus, the releaseability of the recording medium is increased. 
       FIGS. 4A and 4B  are cross-sectional views each illustrating a structure of the induction heating unit  50 .  FIG. 4A  shows a cross-sectional view of the induction heating unit  50  perpendicular to a rotary axis of the heat roller  41  around the center in the longitudinal direction thereof, and  FIG. 4B  shows a cross-sectional view perpendicular to the rotary axis at the end in the longitudinal direction thereof. Specifically, the positions in the longitudinal direction of each of  FIGS. 4A and 4B  correspond respectively to positions A and B in  FIG. 7 , which will be described below. 
     As illustrated in  FIG. 4A  being a cross-sectional view taken at around the center in the longitudinal direction, the induction heating unit  50  includes an excitation coil  51  (which may be referred to simply as a coil  51 ), magnetic cores  52 ,  53 , and  59 , a coil retainer  54 , an aluminum cover  55  serving both as a rectifying member and an electromagnetic wave shield, and a resinous cover  56 . The shape of the aluminum cover  55  in  FIG. 4A  is different from the one in  FIG. 4B  because the cross-sectional position taken along the axial direction is different. Details of the aluminum cover  55  will be described later referring to  FIG. 7  and the like. 
     The magnetic cores disposed to cover the coil  51  include side cores  53  as first cores, arch cores  52  as second cores, and center cores  59  as third cores, and form a magnetic path to concentrate magnetic fluxes generated by the coil  51  to the heat roller  41 . The arch cores  52  are positioned opposed to the circumferential surface of the heat roller  41  and in the back of the coil  51 . The side cores  53  are positioned opposed to the circumferential surface of the heat roller  41  without intermediary of the coil  51  and nearer to the heat roller  41  than to the arch cores  52 . Each center core  59  is positioned at a center end of the arch core  52  and between the arch core  52  and the coil retainer  54 . 
     Preferred materials for magnetic cores such as an arch core  52 , a side core  53 , and a center core  59  are those having less coercive force and high electrical resistivity. Preferred materials for the cores  52 ,  53 , and  59  are ferrite or permalloys such as Mn—Zn ferrite and Ni—Zn ferrite. 
     In addition, as illustrated in  FIG. 5 , the side cores  53  and the center cores  59  are plate-like or bar-shaped cores extending in the longitudinal direction, i.e., in the axial direction of the heat roller  41 . By contrast, as illustrated in  FIG. 4A , the arch cores  52  are arch-shaped along the circumference of the heat roller  41  if seen from the axial direction of the heat roller  41 , and multiple arch cores  52  are disposed at appropriated intervals in the roller longitudinal direction (see  FIG. 5 ), so that the temperature distribution in the longitudinal direction of the heat roller  41  becomes uniform. 
     The coil  51  is formed such that 50 to 500 electrical leads, to which insulation coating is applied, each having a diameter of from approximately 0.05 to 0.2 mm are wound together to form a litz wire, which is wound 5 to 15 times. The litz wire includes a fusion layer on its surface thereof. The fusion layer is solidified by being heated electrically or heated in a constant temperature reservoir, and thus, the shape of the wound coil can be maintained. Alternatively, the litz wire without the fusion layer is wound and can be shaped by press molding. Preferable materials as insulation and covering materials for the base wire of the litz wire include resins such as polyamideimide, polyimide, and the like having both heat resistance and insulation property. According to the present embodiment, use of polyester and polyesterimide having less heat resistance than the conventional ones is intended to improve cooling efficiency of the coil  51 . The finished coil  51  is attached to the coil retainer  54  using a silicon adhesive or the like. The coil retainer  54  requires heat-resistance to a temperature exceeding the fixing temperature and is preferably formed with highly heat-resistant resins such as PET or crystal liquid polymers. 
     The aluminum cover  55  and the resinous cover  56  are disposed at an opposite side of the fixing member (herein, the fixing belt  43 ) and facing the coil retainer  54 . The aluminum cover  55  serves as both an electromagnetic wave shield and a rectifying member. Specifically, the aluminum cover  55  shields electromagnetic wave from the coil  51  and prevents a portion around the coil from being heated due to an influence of the electromagnetic wave. Accordingly, preferred materials as the electromagnetic wave shield are non-magnetic and electrically conductive ones. In the present embodiment, aluminum is used, but alternatively copper, gold, silver, and the like may be used. In addition, the aluminum cover  55  serving as a rectifier adjusts, or rectifies, a flow of air inside the induction heating unit  50 , which will be described later below, and improves efficiency to cool the coil  51 . The aluminum cover  55  as a rectifier is formed of any thermally conductive material so as to promote heat discharge from the coil  51  during operation. To reduce the fluid resistance, a surface of the aluminum cover  55  is preferably smoothed. Accordingly, preferred materials for the rectifier include, other than aluminum used herein, metals such as copper and iron having good thermal conductivity. If the resin is not expected to serve as an electromagnetic shield is not expected, resins having a smoothed surface may be used. The resinous cover  56  serves to house the induction heating unit  50 , and therefore, highly heat-resistant resins such as PET or crystal liquid polymers are preferred. 
     Referring to  FIGS. 6 and 7 , a structure of the induction heating unit  50  will be described in detail. 
       FIG. 6  is a top view illustrating that the coil  51  is fixed on the coil retainer  54  (see  FIGS. 4A and 4B  as cross-sectional views).  FIG. 7  illustrates a cross-sectional view of the induction heating unit  50  seen laterally from a longitudinal direction thereof. 
       FIG. 6  illustrates for simplification the coil  51  as if a plurality of coils is arranged elliptically. In the present embodiment, the coil  51  is formed such that 90 insulated copper wires each having an outer diameter of 0.15 mm are wound together and the thus-wound wire bundle is disposed spirally to cover an entire width of the surface of the coil retainer  54 . The coil retainer  54  covers the heat roller  41  as a heat generator. In addition, the coil  51  is shaped to wind in the roller rotary axis direction along the circumference of the fixing belt  3 . In this case, a central portion of the coil  51  opposite the heat roller  41  and parallel to the rotary axis direction of the heat roller  41  is a linear portion  51   a ; and portions at both ends of the coil  51  each forming substantially a semicircle are curved portions  51   b.    
     As illustrated in  FIG. 7 , the linear portion  51   a  is fixed to the coil retainer  54  and the aluminum cover  55  and the resinous cover  56  are disposed opposite the linear portion  51   a . A cooling fan  57  is disposed at one end of the induction heating unit  50 , and sends air between the coil retainer  54  and covering members (including the aluminum cover  55  and the resinous cover  56 ), so that the coil  51  heated during the operation of the induction heating unit  50  is air-cooled. In the present embodiment, the cooling fan  57  is disposed at one lateral side of the induction heating unit  50 . Alternatively, a flow path such a duct may be disposed between the fan and the induction heating unit  50 . The induction heating unit  50  is air-cooled via the flow path. The cooling fan may be either disposed at the fixing device or mounted to the image forming apparatus. In the latter case, cooled air generated by the cooling fan is sent to the induction heating unit  50  from the image forming apparatus. 
     As illustrated in  FIG. 7 , the aluminum cover  55  serving as the rectifying member has a center portion expanded downward in the direction to the coil retainer  54  which forms a projecting portion  55   a . Accordingly, the coil retainer  54  is closer to the aluminum cover  55  in the center portion than in the end portions. More specifically, the distance between the projecting portion  55   a  and the coil retainer  54  is substantially 60 to 80% of the distance in the end portions. With the configuration as above, speed of an airflow F near the linear portion  51   a  during the operation of the cooling fan  57  is increased in accordance with narrowing of the flow path, thereby improving the cooling effect to the coil  51  as a whole. 
       FIG. 8  illustrates a comparative example 1 for a comparison, in which an aluminum cover  155  is parallel to the coil retainer  54 , so that the cross-sectional area of the airflow path is substantially the same overall in the induction heating unit  50 . As a result, an increase in the airflow speed is not expected and the cooling effect is inferior to that of the present embodiment. 
     In addition, in the first embodiment as illustrated in  FIG. 7 , a front portion  55   b  of the projecting portion  55   a  of the aluminum cover  55  at a side nearer to the cooling fan  57  is formed as a slanted surface connecting the downwardly expanding projecting portion  55   a  with the upper ceiling portion, that is, the resinous cover  56 . Herein, the slanted surface has a slant relative to the coil retainer  54  and has a smoothed surface. Because the front portion  55   b  of the aluminum cover  55  is a slanted surface, the fluid resistance of the aluminum cover  55  relative to the airflow from the cooling fan  57  is reduced, thereby increasing the cooling efficiency. This is because, if the fluid resistance lowers, the speed of the airflow F in the projecting portion  55   a  increases, heat conduction coefficient between the coil  51  and the airflow F increases, thereby increasing the heat discharge. 
     By contrast, in the comparative example 2 as illustrated in  FIG. 9 , if a front surface  255   b  of a central portion  255   a  of an aluminum cover  255  protrudes perpendicularly or with angles near the right angle to the airflow path, the fluid resistance increases, thereby decreasing the cooling efficiency. 
     In addition, as another effect of the reduction of the fluid resistance in the first embodiment, small static pressure of the cooling fan  57  is sufficient to generate a satisfactory airflow even using a small fan, and moreover a general-purpose cooling fan can be used. 
     Further, in the first embodiment, because the front portion  55   b  is disposed opposite the curved portions  51   b , thereby increasing the speed of the airflow F overall in the linear portion  51   a  and increasing the cooling efficiency. In addition, as illustrated in  FIG. 7 , because the air flowing obliquely downward in the curved portions  51   b  nearer to the cooling fan  57 , easily strikes the coil, thereby further increasing the cooling efficiency. 
     A predetermined range of the central portion in which a distance between the coil retainer  54  and the aluminum cover  55  is narrowed may be varied; however, the range preferably at least corresponds to the range of the linear portion  51   a . With this configuration, the linear portion  51   a  can be entirely and efficiently cooled. Alternatively, the central predetermined range can be set larger than the entire length of the linear portion  51   a  so as to stick out in the curved portions  51   b . As illustrated in  FIG. 7 , the projecting portion  55   a  of the aluminum cover  55  extends slightly from linear portion  51   a  to the curved portions  51   b.    
     Further, the front portion  55   b  of the aluminum cover  55  can be formed as a slant extending to an opposite end in the longitudinal direction. A degree of the slant and the position where the slant starts in the slanted surface may be varied. In addition, a rear surface  55   c  of the projecting portion  55   a  of the aluminum cover  55  can be formed as a slanted surface similar to the front portion  55   b  or alternatively as a right angle or a surface with a steep angle. Further, the rear surface  55   c  may be a slanted surface extending up to the opposite end in the longitudinal direction. As such, various forms are possible. 
     Next, an operation of the thus-configured fixing device will be described. 
     The fixing belt  43  rotates in the direction indicated by an arrow in  FIG. 2 , i.e., in the counterclockwise direction. The heat roller  41  is heated by the induction heating unit  50 . Specifically, a high-frequency alternating current of from 10 kHz to 1 MHz is supplied to the coil  51 , so that magnetic fluxes are alternatingly switched in a reciprocal direction in a loop of the coil  51 . Thus, when the alternating magnetic field is formed, an eddy current is generated in the heat roller  41  and joule heat is generated, and the heat roller  41  is induction-heated. The fixing belt  43  is heated by heat from the thus-heated heat roller  41 , the sheet P and the fixing belt  43  are contacted at the nip N, and thus, toner images on the sheet conveyed are heated and fused. 
     Next, cooling operation in the induction heating unit  50  will be described. 
     The cooling fan  57  operates when the fixing device is operated. When the cooling fan  57  rotates, the coil  51  is cooled by air. As described above as to the induction heating unit  50 , because the aluminum cover  55  is disposed as a rectifying member, the distance between the coil retainer  54  to retain the coil  51  and the aluminum cover  55  disposed opposite the coil retainer  54  is shorter in the central portion (that is, the projecting portion  55   a ) than in the end portions. Accordingly, the speed of the airflow F near the linear portion  51   a  increases due to narrowing of the airflow path when the cooling fan  57  operates, and the cooling efficiency of the coil  51  in its entirety can be increased. In addition, because the front portion  55   b  of the aluminum cover  55  is opposed to the curved portion  51   b  of the side of the cooling fan  57 , and the front portion  55   b  is slanted, the fluid resistance of the aluminum cover  55  relative to the airflow when the cooling fan operates is reduced and the airflow strikes on the coil  51  more easily. Thus, the cooling efficiency is further improved. 
       FIG. 10  is a graph illustrating a temperature of the coil when heated in the first embodiment and the first comparative example. In the comparative experiment using the structure as illustrated in  FIG. 7  for the present embodiment and the structure as illustrated in  FIG. 8  for the comparative example, the shape of the aluminum covers ( 55 ,  155 ) alone is different from each other whereas the other structures including the cooling fan are the same. 
     The experiment was conducted as follows. First, electrical power is supplied to the coil  51 , and the temperature of the coil  51  when the surface temperature of the fixing belt  43  is heated up to the set temperature of 170 degrees C. is measured. 
     In the comparative example 1 of  FIG. 8  using the straight aluminum cover  15  is used, the coil temperature increases up to 220 degrees C. By contrast, in the present embodiment 1, the highest temperature remains at 200 degrees C., thereby demonstrating that the temperature rise of the coil  51  in the present embodiment is suppressed by 20 degrees and that the cooling efficiency by the cooling fan  57  is improved compared to the comparative example 1. 
     Thus, in the present first embodiment, the fixing device having an improved cooling efficiency of the coil is realized by an uncomplicated structure and at a low cost without distorting from the outstanding structure of the fixing device. In addition, because the fixing device according to the first embodiment has excellent cooling efficiency, the low-power cooling fan can be used while ensuring that applicable safety standards are met. Additionally, the present embodiment is effective in the cost reduction, power saving, and noise reduction. 
       FIG. 11  illustrates the induction heating unit according to a second embodiment. The induction heating unit according to the second embodiment is configured similarly to that in the first embodiment except that the shape of the aluminum cover  55  is different, and therefore redundant explanations with regard to the similar structure are omitted, and different parts only will be described. 
     In the induction heating unit according to the second embodiment as illustrated in  FIG. 11 , the projecting portion  55   a  as a portion expanding toward the coil is formed entirely as a slanted surface. In addition, the rear surface  55   c  downstream of the projecting portion  55   a  is formed as a right angle or a surface with a steep angle. On the other hand, the leading end of the projecting portion  55   a  reaches the ceiling portion, i.e., the resinous cover  56 . Due to the projecting portion  55   a  formed entirely as a slanted surface, the cooled airflow F introduced by the cooling fan  57  is sent to the coil  51  via the slanted surface of the projecting portion  55   a . As a result, because the cooled air tends to strike on the coil  51  over an entire range of the coil, the cooling efficiency can be improved. In addition, because the angle of the slanted surface of the projecting portion  55   a  is less than the front portion  55   b  according to the first embodiment, the fluid resistance of the aluminum cover  55  is reduced, thereby further improving the cooling efficiency. Further, the rear surface  55   c  may be slanted like the rear surface  55   c  as illustrated in  FIG. 7  according to the first embodiment. 
       FIG. 12  illustrates the induction heating unit according to a third embodiment. The induction heating unit according to the third embodiment is similar to that in the first embodiment except that the shape and function of the aluminum cover  55  and the resinous cover  56  are different, and therefore, the redundant description of the similar portion will be omitted. 
     In the induction heating unit according to the third embodiment as illustrated in  FIG. 12 , not the aluminum cover  55  but the resinous cover  56  serves as a rectifier to rectify a flow of cooled air from the cooling fan  57 . Specifically, the aluminum cover  55  is disposed above and the resinous cover  56  is disposed beneath the aluminum cover  55 . The resinous cover  56  is formed to include a projecting portion  56   a  so as to make the airflow path narrower. Differently from the first and second embodiments, the aluminum cover  55  does not include a projecting portion and is disposed parallel to the coil retainer  54 . 
     The center portion of the resinous cover  56  in the longitudinal direction expands downward to form the projecting portion  56   a , thereby narrowing the airflow path above the linear portion  51   a . With this configuration, speed of the airflow F near the linear portion  51   a  during the operation of the cooling fan  57  is increased as the airflow path is narrowed, thereby improving the cooling efficiency of the coil  51  as a whole. 
     In addition, a front portion  56   b  upstream of a projecting portion  56   a  of the resinous cover  56  is slanted. Herein, the slanted surface means having a slant relative to the opposite coil retainer  54 . The slanted surface also includes a smooth surface. Because the front portion  56   b  of the resinous cover  56  is a slanted surface, the fluid resistance of the resinous cover  56  relative to the airflow from the cooling fan  57  is reduced, thereby increasing the cooling efficiency. 
     In addition, a rear surface  56   c  downstream of the projecting portion  56   a  of the resinous cover  56  is formed as a slanted surface; however, similar to the rear surface  55   c  of the aluminum cover  55  as illustrated in  FIG. 11  according to the second embodiment, the rear surface  56   c  may be formed as a right angle or a surface with a steep angle. Further, the rear surface  56   c  may be a slanted surface extending up to the opposite end in the longitudinal direction. As such, various forms are possible. 
       FIG. 13  illustrates the induction heating unit according to a fourth embodiment. The induction heating unit according to the fourth embodiment is similar to that in the third embodiment except that the shape of the resinous cover  56  is different, and therefore, the redundant description of the similar portion will be omitted. 
     In the induction heating unit according to the fourth embodiment as illustrated in  FIG. 13 , the projecting portion  56   a  of the resinous cover  56  is formed entirely as a slanted surface. The function that the projecting portion  56   a  of the resinous cover  56  according to the fourth embodiment is the same as that performed by the projecting portion  55   a  of the aluminum cover  55  according to the second embodiment as illustrated in  FIG. 11 . Specifically, the cooled airflow F introduced by the cooling fan  57  is sent to the coil  51  via the slanted surface of the projecting portion  56   a  of the resinous cover  56 . As a result, because the cooled air tends to strike on the coil  51  over an entire range of the coil, the cooling efficiency is improved. In addition, because the angle of the slanted surface of the coil-approaching surface  56   a  is less than the front portion  56   b  of the third embodiment, the fluid resistance of the resinous cover  56  becomes less, thereby further improving the cooling efficiency. 
     Next, a description will be given of a fifth embodiment, applied to a fixing device employing a heat roll method. 
       FIG. 14  shows a fixing device including a fixing roller  45  serving as a fixing member and the induction heating unit  50  which heats the fixing roller  45 . Except that the fixing roller  45  serves as the fixing device, the structure of the fixing device according to the fifth embodiment is the same as that of the fixing device as illustrated in  FIG. 2 . In the structure according to the fifth embodiment, the fixing roller  45  serves as the fixing member and also as the heat-generating member, because the fixing roller  45  is heated by the induction heating unit  50  and generates heat. 
     The structure and the operation of the induction heating unit  50  for use in the fifth embodiment are the same as those described in the first embodiment, and various configurations as described in the first to fourth embodiments may similarly be applied to the fifth embodiment, and therefore, the redundant description thereof will be omitted. 
     Specifically, the fixing roller  45  according to the fifth embodiment has an outside diameter of from 30 to 40 mm and includes an elastic layer  45   b , a heat-generating layer  45   c , and a release layer (not shown) laminated on a metal core  45   a . The fixing roller  45  rotates in a counterclockwise direction in the figure, is heated by induction heating, and fuses the toner image carried on a recording sheet conveyed to the fixing nip portion. 
     As described above, the release layer, not shown, is formed as the topmost layer of the fixing roller  45 . The release layer may be formed of fluorine resins such as: polytetrafluoroethylene (PTFE); tetrafluoroethylene-perfluoroalkyl vinylether copolymer (PFA); and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or mixture of these resins, or heat-resistant resins dispersed with above resins. The thickness of the release layer ranges from 5 to 50 μm, and more preferably from 10 to 30 μm. Such a range ensures both good toner releaseability from the fixing roller  45  and flexibility of the fixing roller  45 . 
     The heat roller  45  includes the heat-generating layer  45   c  formed of Cu with a thickness of from 3 to 15 μm on the surface thereof, to thus improve the heat-generating effect. In this case, nickel coating is preferably applied on the surface of the Cu layer for preventing corrosion. 
     The elastic layer  45   b  of the fixing roller  45  can employ elastic members such as fluororubber, silicon rubber, and fluorosilicon rubber. Because the elastic layer  45   b  is employed in the fixing roller  45 , bending of the fixing roller  45  is allowed and a width of the nip area can be increased. In addition, by lowering the stiffness of the fixing roller  45  than that of the pressure roller  44 , sheet dischargeability and separability may be improved. In addition, by forming the elastic layer  45   b  with a sponge rubber, heat generated by the heat-generating layer  45   c  can be insulated, and the elastic layer and the release layer opposing to the surface of the fixing roller can be heated rapidly. Thus, the surface of the fixing roller is rapidly heated and reaches a temperature necessary for fixing and heat supply can be enough even though heat is absorbed by the recording medium. With such a configuration, an optimal nip area can be formed and heat-generating in the heat-generating layer  45   c  can be insulated, so that heat is prevented from being transmitted to an interior of the fixing roller  45 . The elastic layer  45   b  according to the present embodiment employs foamed silicon rubber having an approximate thickness of 9 mm. As a result, heat of the heat-generating layer  45   c  disposed on the surface layer of the fixing roller  45  does not flow into the fixing roller  45  easily, and effective heating can be performed. 
     The metal core  45   a  of the fixing roller  45  has sufficient stiffness to bear a load given to the fixing roller  45  for forming a nip area. For that purpose, the metal core employs metals such as aluminum and iron. The metal core  45   a  may also employ non-magnetic, insulating materials such as ceramics so as not to affect the induction heating. The metal core  45   a  according to the fifth embodiment employs aluminum. According to the present structure, an external diameter of the core metal  45   a  is 22 mm and a thickness thereof is 2.0 mm. Stiffness to bear the load applied to the fixing roller  45  is required for the core metal  45   a  so that the thickness of the core metal  45   a  is 2.0 mm. 
       FIG. 15  illustrates an induction heating unit configured alternatively according to a sixth embodiment. 
     The induction heating unit  150  according to the sixth embodiment includes the excitation coil  51 , the coil retainer  54 , the resinous cover  56 , the cooling fan  57 , and an aluminum rectifying member  58 . The aluminum rectifying member  58  is not opposed to the linear portion  51   a  of the excitation coil, but is disposed at the upstream end of the induction heating unit  150  near the side of the cooling fan  57 . 
     The aluminum rectifying member  58  includes a slanted surface  58   a  protruding to a space below the resinous cover  56 . The aluminum rectifying member  58  is not opposed to the linear portion  51   a  of the excitation coil, but includes a slanted surface  58   a  extending to the side of the coil from the end of the cooling fan  57 , thereby narrowing the airflow path. As a result, because the speed of the airflow F flowing through the coil linear portion  51   a  increases, the coil cooling efficiency is improved. 
     As described above, the fixing device according to the first to sixth embodiments includes a structure in which a distance between the coil retainer and the cover member is narrowed in the center portion covering the predetermined area in the longitudinal direction of the excitation coil and in the fixing member rotary axis direction, or alternatively, includes a rectifying member extending from the cover member to the coil retainer and narrowing the airflow path formed between the cover member and the coil retainer. As a result, the speed of the cooled air passing through the coil disposed position increases, thereby improving cooling efficiency of the coil. Accordingly, the rotation number of the cooling fan can be reduced and the compact-size cooling fan can be realized, so that the cost and the energy consumption can be reduced and the noise can be reduced. 
     In addition, the range where the distance between the coil retainer and the cover member is shortened is set to at least an area corresponding to the linear portion of the excitation coil, so that the linear portion of the coil being a greater heat-generating area can be effectively cooled. 
     In addition, because the distance to the coil retainer is shortened by the projecting portion disposed on the cover member, shortening of the airflow path can be realized with an uncomplicated structure. 
     Further, because the portion near the cooled air inlet of the projecting portion is formed as a slanted surface, the fluid resistance of an inside of the induction heating unit can be reduced, thereby further improving the coil cooling efficiency. 
     Furthermore, because the slanted surface is disposed facing the curved portion of the excitation coil, the cooling air from a ventilation means can effectively be introduced to the coil surface. 
     Further, because the projecting portion is entirely formed as a slanted surface, the fluid resistance of an inside of the induction heating unit can be further reduced, thereby further improving the coil cooling efficiency. 
     In addition, because the shield member to protect against electromagnetic radiation is made a cover member, use of the shield member may improve the cooling efficiency of the coil. 
     In addition, because the housing of the induction heating unit is made a cover member, use of the unit housing may improve the cooling efficiency of the coil. 
     Furthermore, because the rectifying member protruding from the cover member is disposed near an end of the cooled air inlet, the cooled air from the ventilation means can be effectively introduced to the coil surface. 
     Further, because the surface at the cooled air inlet of the rectifying member is formed as a slanted surface, the fluid resistance can be prevented from increasing. 
     Next, a seventh embodiment according to the present invention will be described with reference to  FIGS. 16 through 30 . A structure and operation of the image forming apparatus as a whole is the same as those described referring to  FIG. 1  and therefore, the explanation thereof will be omitted. 
     Hereinafter, operation of the thus-configured fixing device will be described. 
     The fixing belt  43  rotates in the direction indicated by an arrow in  FIG. 16 , i.e., in the counterclockwise direction. The heat roller  41  is heated by an induction heating unit  150 . Specifically, because a high-frequency alternating current of 10 kHz to 1 MHz is supplied to an excitation coil  151 , magnetic fluxes are formed in a loop of the excitation coil  151  so as to be alternatingly switched in a reciprocal direction. Thus, when the alternating magnetic field is formed, an eddy current is generated in the heat roller  41  and joule heat is generated, and the heat roller  41  is induction-heated. The fixing belt  43  is heated by heat from the thus-heated heat roller  41 , the sheet P conveyed and the fixing belt  43  are contacted at the nip N, and thus, toner images on the sheet is heated and fused. 
     If the induction heating efficiency is improved, a surface temperature of the fixing belt  43  is rapidly increased, and the heat rise property becomes very optimal. Heat rise property indicates a rise time to reach a temperature necessary for the fixing belt  43  to fix the toner. A shorter rise time is regarded as optimal. 
     In addition, if the heating efficiency is improved, power necessary to print the sheet is also reduced, thereby providing a low-energy-consumption image forming apparatus. 
     When the heat roller is being heated, because the high-frequency alternating current is supplied to the excitation coil  151 , the coil itself is heated by the joule heat and the temperature of the excitation coil  151  increases. In addition, in the seventh embodiment, the excitation coil  151  is disposed as close to the heat roller  41  as possible. As a result, the temperature of the excitation coil  151  increases due to the radiation heat from the heat roller  41 . 
     Herein, if the temperature of the excitation coil  151  increases, the electrical resistance of the excitation coil  151  increases due to the relation between the electrical resistance of the metal and the temperature coefficient, and the heat-generating efficiency decreases. Accordingly, when a printing operation of the electrophotographic apparatus continues from an initial operation, the temperature of the excitation coil increases, and by contrast, the heat-generating efficiency gradually is reduced. To prevent this decrease in efficiency and any accident such as breaking of the excitation coil, a cooling fan is used to cool the excitation coil by airflow. In the present embodiment, as described later below, a cooling fan  161  is provided to send cooled air into the induction heating unit  150  to thereby cool the excitation coil  151 . In the present embodiment, the cooling efficiency of the excitation coil by the cooled airflow is improved, which will be described below. 
       FIG. 17  illustrates a structure of an induction heating unit  150 .  FIG. 17  shows a cross-sectional view of the induction heating unit  150  around the end in the longitudinal direction thereof, seen from an axial direction of the heat roller  41 . Specifically, the cross-section in  FIG. 17  is taken along line A-A in  FIG. 20 , which will be described later. 
     As illustrated in  FIG. 17 , the induction heating unit  150  according to the present embodiment includes excitation coil  151 , arch cores  152 , center cores  153 , side cores  154 , a case  155 , a cover  156 , and a rectifying member  157 . The case  155  serves also as a coil retainer on which the excitation coil  151  are disposed. 
     Core members including the arch cores  152 , the center cores  153 , and the side cores  154 , form a magnetic path in which the magnetic flux generated by the excitation coil  151  is focused to the heat generating member such as the heat roller  41 . 
     Multiple arch cores  152  are disposed at appropriate intervals in the roller longitudinal direction (see  FIG. 19 ), so that the temperature distribution in the longitudinal direction of the heat roller  41  becomes uniform. Because the structure and material of the magnetic cores are similar to those in the first embodiment, redundant descriptions are omitted. 
     The case  155  is covered by the cover  156  to prevent contact with parts inside the induction heating unit  150  mistakenly. The cover  156  should be heat-resistant and is preferably formed with highly heat-resistant resins such as PET or crystal liquid polymers, if resins are employed. In the seventh embodiment, the cover  156  employs an aluminum material which also serves as electromagnetic shielding to avoid leakage from the magnetic field. 
     Referring now to  FIG. 18 , the structure of the excitation coil  151  will be described in detail. 
     As described above, the excitation coil is formed such that the surface-insulated litz wire is wound around multiple times in the rectangle shape extending in the axial direction of the heat roller  41  as illustrated in  FIG. 18 . In addition, the thus-wound excitation coil  151  is curved in the arch shape along an outer circumference of the heat roller  41  seen from the axial direction of the heat roller  41 . Then, in the present embodiment, center cores  153  are disposed, at a portion corresponding to the interior of the rectangle shape, along the axial direction of the heat roller  41 . 
       FIG. 18  schematically illustrates the excitation coil  151  wound in the rectangle shape and  FIG. 19  obliquely illustrates an embodied example. 
     As illustrated in  FIG. 19 , the rectangle excitation coil  151  with round corners is attached on an upper surface of the case  155  which also serves as a coil retainer. The case  155  is exposed at an internal side of the excitation coil  151 . Further, as illustrated in  FIG. 19 , two center cores  153  are disposed on the internal side of the excitation coil  151  along the axial direction of the heat roller  41 . Furthermore, the side cores  154  are disposed at an external side of the excitation coil  151  along two longitudinal sides of the rectangle shape extending in the axial direction of the heat roller  41 . Multiple arch cores  152  are disposed at intervals each other so as to cover the excitation coil  151  from above. 
     In the present embodiment, the cover  156  is fixed with screws to the case  155  by three bosses  158  positioned at an internal side relative to the excitation coil  151 , that is, at both ends and a center position. The bosses  158  are integrally formed on the case  155 . In addition, a circular opening  160  is disposed at a substantially center of the internal side in the longitudinal direction of the excitation coil  151 . A non-contact type temperature sensor, not shown, to measure a temperature of the heat roller  41  is disposed at the round opening  160 . A cylindrical protection wall  159  is disposed around the circumference of the opening  160 . The protection wall  159  is integrally formed on the case  155 . 
     Further, as illustrated in  FIG. 20 , the rectifying member  157  is disposed inside the excitation coil  151  along the longitudinal direction of the heat roller  41 . In the present embodiment, the rectifying member  157  is integrally formed with the case  155 ; however, the rectifying member  157  may be formed separately from the case  155 . The rectifying member  157  has a shape like a wall and has a rectangle shape with round both ends in the longitudinal direction if seen from above. In  FIGS. 18 and 19 , the rectifying member  157  is not illustrated unintentionally. In  FIG. 20 , the bosses  158  and the protection wall  159  are not illustrated. In addition, the bosses  158  disposed at both ends can be integrally formed with the rectifying member  157 . Further, in the present seventh embodiment, the central boss  158  and the protection wall  159  are disposed inside the rectifying member  157 . 
     As described above, the rectifying member  157  is disposed in the wall shape and, in  FIG. 17  that is a cross-sectional view of the induction heating unit  150  taken along the Line A-A near the end in the longitudinal direction of the induction heating unit  150  in  FIG. 20 , an external end surface of the rectifying member  157  is illustrated as a rectangle. In  FIG. 21  that is a cross-sectional view along the B-B line in  FIG. 20 , the rectifying member  157  is illustrated as two wall members. 
     As is known from  FIG. 17 , the rectifying member  157  is so disposed as to divide into two (laterally in  FIG. 17 ) a space formed by the excitation coil mounting surface of the case  155  and an interior wall of the cover  156  opposed to the coil mounting surface. In the present embodiment, the rectifying member  157  is integrally formed with the case  155  so as to extend from an upper surface of the case  155  and there is no gap between the rectifying member  157  and the case  155 . Further, when the cover  156  is fixed to the case  155  with screws, the cover  156  is contacted an upper edge of the rectifying member  157 , so that there is no gap between the rectifying member  157  and the cover  156 . 
     As a result, because inside the induction heating unit  150  the rectifying member  157  is so disposed as to divide into two the space in which the excitation coil  151  are disposed, an airflow when cooled air is sent from one end in the longitudinal direction of the induction heating unit  150  is similarly divided into two by the rectifying member  157 . 
     The rectifying member  157  is preferably formed with highly heat-resistant resins such as PET or crystal liquid polymers. In the present embodiment, the liquid crystal polymer that is the same material used for the case  155  is used for the rectifying member  157 . 
     As will be described later, the rectifying member is used to rectify an airflow inside the induction heating unit to improve cooling efficiency, it can be formed with any thermally conductive materials, so that heat discharge from the excitation coil can be promoted during the operation. To reduce the fluid resistance, a surface of the rectifying member  157  is preferably smoothed. Accordingly, preferred materials for the rectifying member  157  include, other than materials cites herein, metals such as aluminum, copper, and iron, and resins having a smoothed surface may be used. 
       FIG. 22  schematically illustrates a function of the rectifying member  157  according to the seventh embodiment. 
     In  FIG. 22 , a cooling fan  161  is disposed at one end in the longitudinal direction of the induction heating unit  150  (the cover  156  is omitted in the same figure). The cooling fan  161  sends cooled air into the induction heating unit. In the present embodiment, the cooling fan  161  is disposed at one lateral side of the induction heating unit  150 . Alternatively, an airflow path such as a duct is formed to connect the cooling fan  161  and the induction heating unit  150  and the induction heating unit  150  is air-cooled via the airflow path. 
     As illustrated in  FIG. 22  with a bold arrow, the cooled air sent by the cooling fan  161  into the induction heating unit  150  first cools a curved portion as a shorter side of the excitation coil  151 , that is, a curved portion connecting two linear portions of the excitation coil  151 , and then, the thus-sent cooled air strikes on the rectifying member  157  and is branched into two currents. The two currents flow along the longer sides of the excitation coil  151 . The air current flows along the outer side of the rectifying member  157  as illustrated in the cross-sectional view in  FIG. 21  and the cooled air flows along the excitation coil  151 . In this case, the air does not flow a portion between two upright members of the rectifying member  157 , that is, an inner portion of the excitation coil  151  where no coil exists. 
       FIG. 23  schematically illustrates the airflow of the induction heating unit according to a conventional structure without a rectifying member. In the conventional example, the cooled air sent from the cooling fan into the unit flows linearly as is and passes through the unit. The air flows along the inner side of the excitation coil  151  where no coil exists. Much air flows along the inner side of the excitation coil  151  because there are few obstacles such as an arch coil  152  to disturb the airflow. 
     However, the excitation coil needs to be cooled most in the induction heating unit, safety is secured by reducing a temperature of the excitation coil, and the heating efficiency can be improved by decreasing the coil resistance. The rectifying member  157  according to the present invention can focus the airflow around the excitation coil  151  and increase the speed of the airflow, thereby improving the cooling efficiency of the excitation coil  151 . 
     By contrast, because the conventional induction heating unit causes the cooled air flowing inside the unit to evenly flow into both the coil-equipped portion and the center portion where no coil exists, the cooling efficiency is inferior to that in the present embodiment. 
     As the amount of airflow increases, the cooling efficiency rises. This is because the heat conduction coefficient between the excitation coil  151  and the airflow increases, and the discharged heat amount to air increases. 
     Herein, as illustrated in  FIG. 22 , a distance L 1  between the rectifying member  157  and the curved portion of the excitation coil  151  at upstream of the cooled air path nearer to the cooling fan  161  is preferably set to shorter than a distance L 2  at an opposite side downstream in the cooled air path. More specifically, as illustrated in  FIG. 22 , the distance between the rectifying member  157  and the excitation coil  151  is defined as follows: L 1 &lt;L 2 , wherein L 1  shows a distance in the upstream of the cooled air path and L 2  shows a distance in the downstream of the cooled air path. The above inequality stands because, upstream, the end portion of the rectifying member  157  is set closer to the curved portion of the excitation coil  151  and sends the airflow divided by the rectifying member  157  into two along the coil curved portion, thereby effectively cooling the coil curved portion. 
     On the other hand, because much distance is required for the air currents divided by the rectifying member  157  to get together in the downstream of the cooled air path, if the end portion of the rectifying member  157  approaches the coil curved portion, cooling efficiency is degraded. Therefore, by making the distance L 2  greater than L 1 , the coil curved portion can be effectively cooled by the air joined-together in the downstream of the rectifying member  157 . 
     In a comparative experiment, the seventh embodiment has been compared to the conventional one as described above (see  FIG. 10 ). In the comparative experiment, the seventh embodiment, which has been described referring to  FIGS. 16 to 22 , and a third comparative example described referring to  FIG. 23  have been used. The difference between the two resides in the presence or absence of the rectifying member  157 ; otherwise, the remaining structure is the same. The experiment was conducted as follows. 
     First, electrical power is supplied to the coil  151 , and, after the surface temperature of the fixing belt  43  is heated up to the set temperature of 170 degrees C., the set temperature is held and the temperature of the coil  151  is measured when printing has been done continuously. The temperature of the excitation coil  151  was measured at a position in which the temperature becomes highest at an outlet of the airflow. In the comparative example, the temperature of the excitation coil temperature has increased up to 220 degrees C. By contrast, in the present seventh embodiment, the highest temperature remains at 200 degrees C. As a result, it can be seen that the temperature rise of the coil  151  in the present seventh embodiment is suppressed by 20 degrees and that the cooling efficiency is improved compared to the comparative example 3. 
     That is to say, the present invention provides an optimal fixing device capable of effectively cooling the coil with uncomplicated structure. With such a structure, the low-power cooling fan can be used while conforming to applicable safety standards, and a low-cost, energy-saving, and low-noise induction heating unit can therefore be expected. In addition, if the heating efficiency is improved by cooling the excitation coil, power necessary to print the sheet is also reduced. 
     In addition, the rectifying member is preferably disposed inside or at an internal circumference of the excitation coil. If the rectifying member is disposed at an external circumference of the excitation coil, the curved portion at an end of the coil cannot be cooled effectively. A description is given below of the reason thereof. 
       FIG. 24A  illustrates a coil curved portion in the seventh embodiment (Example 7).  FIG. 24B  illustrates a comparative example 4 including another rectifying member  257  disposed outside the excitation coil  151 . Marks A to D shown in  FIGS. 24A and 24B  correspond to positions of the excitation coil  151 .  FIG. 25  is a graph to show temperature curves measured at corresponding positions A to D when the induction heating unit is heated. 
     By disposing the rectifying member  257  outside the excitation coil  151 , the airflow is earlier sent to a linear portion of the excitation coil  151 , so that the temperature at C is higher than that at A; however, because the airflow is disturbed in the downstream path, the temperature at D at the end of the curved portion is not cooled effectively. 
     Even a partial heat rise may cause breakage of the excitation coil, and therefore, the rectifying member is preferably disposed inside the excitation coil to cool the excitation coil entirely. 
     In the seventh embodiment, the end surface of the rectifying member  157  opposed to the curved portion of the coil end is curved protruding upstream. This is to reduce the fluid resistance of the rectifying member  157  relative to the airflow. When the fluid resistance is reduced, the speed of the airflow increases and the cooling efficiency is further improved. 
       FIG. 26  shows an example of the rectifying member having a wall confronting the airflow, which is not preferable due to an increase of the fluid resistance. By contrast, as illustrated in  FIG. 27 , the rectifying member  157 B having a triangular leading end upstream is preferable because it reduces fluid resistance. The fluid resistance decreases due to the leading end having an acute angle. 
     In addition, because the rectifying member  157  is integrally formed with the case  155 , this integrity serves as a rib to increase stiffness of the case  155 . 
       FIGS. 28A to 29B  schematically illustrate an intended design of the induction heating unit in the background art and an actually molded product in the present embodiment, respectively.  FIGS. 28A to 29B  each illustrate an induction heating unit seen from the longitudinal direction of the unit. 
     As seen from  FIG. 28B , in the conventional method without using the rectifying member serving also as the rib, the molded product may be curved despite the intended design. When the case  155  is curved, the distance between the excitation coil and the heat roller  41  is away, thereby decreasing the heat-generating efficiency. When the case  155  is made thicker, the distance between the excitation coil and the heat roller  41  is made greater, resulting in reduction of the heat-generating efficiency. 
     On the other hand, in the embodiment as illustrated in  FIGS. 29A and 29B , the rectifying member  157  disposed at an inner circumference of the excitation coil, is used integrally with the case  155 . As a result, there is provided a rib at a center in the longitudinal direction of the case  155 , thereby strengthening the case  155 . Provision of the rib may increase stiffness, thereby reducing the curve. Further, the other portion of the case can be formed with a thinner wall. Reducing the curve and thinning the wall allows the induction heating unit to be closer to the heat roller  41 , and heat-generating efficiency of the induction heating can be improved. 
     The present invention is not limited to the fixing device employing a fixing belt method, but can be applied to the fixing device employing a heat roll method. Hereinafter, a description will be given of an eighth embodiment applied to the fixing device employing the heat roll method. 
       FIG. 30  shows a fixing device including a fixing roller  45  serving as a fixing member and the induction heating unit  50  to heat the fixing roller  45  according to the eighth embodiment. Except that the fixing roller  45  is used as the fixing member, the fixing device according to the eighth embodiment is similar to that in the seventh embodiment as illustrated in  FIG. 16 . In the structure according to the eighth embodiment, the fixing roller  45  serves as the fixing member and also as the heat-generating member, because the fixing roller  45  is heated by the induction heating unit  150  and generates heat. 
     The structure and operation of the induction heating unit  150  used in the eighth embodiment are the same as those described in the seventh embodiment including the rectifying member  157 , and therefore, the redundant description thereof will be omitted. Furthermore, because the fixing roller  45  in the eighth embodiment is the same as that in the seventh embodiment, the redundant description thereof is omitted. 
     The fixing device as described in the above seventh and eighth embodiments includes a rectifying member, to divide the air flowing into the induction heating unit  150  into two currents, disposed inside the induction heating unit  150  between the excitation coil-mounted surface and the opposite surface thereof and at an inner side of the excitation coil, so that the speed of the airflow of the cooled air near the excitation coil is increased and the cooling efficiency can be improved without increasing the power of the cooling fan, so that the induction heating unit is energy-saving. As the coil temperature is further reduced, the heating efficiency of the excitation coil is improved more. Thus, a low-cost and highly heating efficient fixing device can be provided. In addition, because a high cooling efficiency can be obtained with an uncomplicated configuration of providing the rectifying member, the cost rise can be minimized. 
     Further, the rectifying member is disposed along the longitudinal direction of the induction heating unit so as to send the airflow divided in two along the linear, longitudinal direction of the excitation coil, thereby being capable of efficiently cooling the longer side portion of the excitation coil having a greater thermal energy. 
     The end surface upstream in the airflow path of the rectifying member can be a curved or sharply-angled shape, the fluid resistance of the rectifying member is reduced and the cooling efficiency can be further improved. 
     The distance between the rectifying member and the curved surface of the excitation coil in the upstream of the airflow path is shorter than that in the downstream in the airflow path. Therefore, the airflow divided into two by the rectifying member is sent along the coil curved portion, thereby effectively cooling the coil curved portion. In the downstream of the airflow path, the airflow joined together in the downstream of the rectifying member is sent along the coil curved portion, thereby effectively cooling the coil curved portion. 
     When the rectifying member is formed of any materials with excellent thermal conductivity, heat discharge from the excitation coil during the operation can be promoted. 
     Further, because the rectifying member is used also as a reinforcement rib, the strength of the case retaining the excitation coil can be enforced, deformation of the case can be suppressed and the thickness of the case can be smaller. 
     Heretofore, the present invention has been described with reference to drawings, but is not limited to the aforementioned embodiments alone, and can be varied in the scope. Further, a size and shape of the rectifying member can be varied within the scope. For example, the shapely-angled shape of the rectifying member in  FIG. 27  is described such that the leading end is closed and the airflow is divided into two. However, as long as the structure to send the air substantially effectively is realized, the leading end of the rectifying member can be formed otherwise even though the airflow is not divided into two perfectly. Materials for the rectifying member are also variable. 
     Additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.