JP5210958B2 - Fixing apparatus and image forming apparatus equipped with the same - Google Patents

Description

  The present invention relates to a fixing device that heats and melts unfixed toner on a sheet while passing the nip between a heated roller pair or a heating belt and a roller on a sheet carrying a toner image, and an image forming apparatus equipped with the fixing device It is about.

  In recent years, in this type of image forming apparatus, a belt system capable of setting a small heat capacity has attracted attention because of demands for shortening the warm-up time in the fixing device and saving energy. In recent years, the electromagnetic induction heating method (IH) that has the potential for rapid heating and high-efficiency heating has attracted attention. From the viewpoint of energy saving when fixing color images, electromagnetic induction heating is combined with the belt method. Many products have been commercialized. When combining the belt method and electromagnetic induction heating, many cases are adopted in which electromagnetic induction devices are arranged outside the belt because of the coil layout and ease of cooling, and the advantage that the belt can be directly heated (so-called). Envelope IH).

  In the electromagnetic induction heating method described above, various technologies have been developed to prevent overheating in the non-sheet passing area according to the width of the sheet size (sheet passing width) that passes through the fixing device. In particular, a technique having a size switching means in the outer package IH is disclosed (for example, see Patent Document 1). Specifically, a center core that forms a magnetic path around the coil is provided, and when the core rotates, the heat roller can be induction-heated by the magnetic field generated by the coil, and the heat roller in the non-sheet passing area and the sheet passing area. A difference in the amount of heat generated can be provided.

International Publication No. 05/038535 Pamphlet

However, the above-described conventional technique has a problem that it is impossible to achieve both heat generation performance and magnetic shielding performance and securing the arrangement space.
Specifically, the former heat generation performance can be evaluated by the length of the warm-up time, and it is desirable that the distance between the coil and the heat roller is short. On the other hand, the latter magnetic shielding performance can be evaluated by the temperature of the heat roller in the non-sheet passing region, and it is desirable that the distance between the center core and the heat roller is short.

  On the other hand, an actual coil is mounted on a coil holding portion (coil bobbin). According to the above-described conventional technique, the bobbin has an envelope circular surface whose center position is concentric with the rotation center position of the heat roller. The bobbin is disposed between the coil and the heat roller. That is, even if the coil position is desired to be close to the heat roller, contact between the bobbin and the heat roller must be avoided. In order to secure the strength of the bobbin, it is not preferable to form the envelope circular portion with a thin wall.

  Furthermore, even if it is desired to bring the center core position closer to the heat roller, the bobbin is still in the way. In this case, it may be possible to remove the confronting part of the bobbin with the center core or the heat roller. However, in this case, the strength of the bobbin is also lowered, and the coil is easily affected by the radiant heat from the heat roller. There is a new problem.

As described above, the point where the coil and the center core are brought close to the heat roller and the point where the coil bobbin is provided are similar in trade-off, but there should be an arrangement satisfying both of them as much as possible. However, in the above conventional technique, no special consideration is given in this respect.
Accordingly, an object of the present invention is to provide a fixing device that solves the above-described problems and satisfies both performance and arrangement, and an image forming apparatus equipped with the fixing device.

  According to a first aspect of the present invention for achieving the above object, a sheet on which a toner image is transferred in an image forming unit is sandwiched and conveyed between a rotating heating member and a pressure member. A fixing device for fixing a toner image onto a sheet by heat from the coil, which is disposed along the outer surface of the heating member and generates a magnetic field for inductively heating the heating member, and an envelope facing the outer surface of the heating member A coil holding portion on which a coil is placed, a coil holding portion on which a coil is placed, and a heating member arranged opposite to the heating member, and a first magnetic material configured to form a magnetic path around the coil. A core and a second core that is provided between the first core and the heating member as viewed in the direction in which the magnetic field is generated by the coil, and is made of a magnetic material so as to form a magnetic path together with the first core; Along the outer surface of the second core A shield member made of a non-magnetic metal to shield the magnetism within the magnetic field generated by the coil, a shield position where the shield member shields the magnetism with the rotation of the second core, and the passage of magnetism. A magnetic shielding amount adjusting means for switching to an allowable retreat position, the center position of the envelope circle of the coil holding portion is different from the rotation center position of the heating member, and the position of the second core is maintained or the heating member On the other hand, the coil holding part is disposed so as to be away from the heating member and close to the second core.

  According to the first aspect of the present invention, a system (outer packaging IH) in which a heating member rotating by a magnetic field generated by a coil is inductively heated to heat and melt the toner image is adopted, and the first core generates the coil. It is arranged around the coil to form a magnetic path that guides the magnetic field. Further, when the magnetic shielding amount adjusting means rotates the second core and moves the shielding member to the retracted position, the magnetic field generated by the coil is guided to the first core and the second core, and the eddy current flows to the heating member. Is generated and magnetic induction heating is performed. On the other hand, when the magnetic shielding amount adjusting means rotates the second core and moves the shielding member to the shielding position, the magnetic resistance in the magnetic path increases, the magnetic field strength decreases, and the heating value of the heating member decreases. .

  Here, unlike the conventional case, the center position of the envelope circle of the coil holding portion and the rotation center position of the heating member are not concentric. In comparison, the second core moves away from the heating member and approaches the second core, and the second core is not separated from the heating member. As a result, the coil holding portion can be disposed between the second core and the heating member while ensuring the proximity state between the second core and the heating member, and the heat generation performance and magnetic shielding performance are disposed. It is possible to achieve a balance with space.

According to a second aspect of the present invention, in the configuration of the first aspect of the present invention, the coil holding portion is a flat curved surface portion formed by an envelope circular surface, and is opposed to both the heating member and the second core. And a surface portion.
According to the second invention, in addition to the action of the first invention, the coil holding portion is further brought close to the second core, and the flat facing surface portion of the coil holding portion is the second core. Since they face each other, the coil holding portion can be reliably closed between the second core and the heating member while ensuring the proximity state between the second core and the heating member. As a result, the strength of the coil holding portion is improved as compared with the case where there is no facing surface portion, and the coil is hardly affected by the radiant heat from the heating member.

According to a third aspect of the present invention, in the configuration of the first aspect, the coil holding portion is opposed to both the curved surface portion formed by the envelope circular surface, the heating member, and the second core, and compared to the curved surface portion. And having a thin and opposite surface portion.
According to the third invention, in addition to the action of the first invention, the coil holding part is further brought close to the second core, and the thin opposing surface part of the coil holding part is used as the second core. Since they face each other, the coil holding portion can be reliably closed between the second core and the heating member while ensuring the proximity state between the second core and the heating member. Therefore, the strength of the coil holding portion is improved as compared with the case where there is no facing surface portion, and the coil is hardly affected by the radiant heat from the heating member.

According to a fourth aspect of the present invention, in the configuration of the first to third aspects, a heating belt is wound around the heating member, and a second surface facing the coil holding portion from the surface of the heating belt facing the coil holding portion. When a linear distance L (mm) to the surface of the core is satisfied, 3.0 ≦ L ≦ 4.0 is satisfied.
According to the fourth invention, in addition to the effects of the first to third inventions, the linear distance L from the surface of the heating belt to the surface of the second core is set in the range of 3.0 mm to 4.0 mm. Then, the second core and the heating belt can be brought close to each other, and the magnetic shielding performance can be secured. Moreover, a coil holding part can be arrange | positioned between these 2nd cores and a heating belt, and also the distance of the coil on a coil holding part and a heating belt becomes favorable, and can maintain heat generating performance reliably.

According to a fifth invention, in the configuration of the second or third invention, a straight line from a virtual envelope circle defined by a lower surface facing the heating member of the curved surface portion to a surface of the second core facing the facing surface portion When the distance is f (mm), 0 ≦ f ≦ 1.0 is satisfied.
According to the fifth invention, in addition to the operation of the second and third inventions, in addition to the case where the surface of the second core is in contact with the virtual envelope circle with the curved surface portion extended, the envelope circle 1. Even when the distance is 0 mm, the coil holding portion can be disposed between the second core and the heating member while securing the proximity state between the second core and the heating member.

According to a sixth aspect of the invention, there is provided an image forming apparatus in which the first to fifth fixing devices are mounted and the toner image formed by the image forming unit is fixed on the sheet by using the fixing devices.
According to the sixth aspect, in addition to the effects of the first to fifth aspects, the heat generation performance and the magnetic shielding performance are satisfied, so that a good toner image is formed. As a result, the reliability of the image forming apparatus is improved. Will improve.

  According to the present invention, since the center position of the envelope circle of the coil holding portion is different from the rotation center position of the heating member, the coil holding portion is separated from the heating member, while the second core is not separated from the heating member. In addition, it is possible to provide a fixing device capable of achieving both the performance of magnetic shielding and securing the arrangement space, and an image forming apparatus equipped with the fixing device.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment. FIG. 3 is a longitudinal sectional view showing a structural example of a fixing unit. It is a top view of the periphery of a center core. It is explanatory drawing of a center core and a shielding member. It is a figure which shows the operation example accompanying rotation of a center core. It is the experimental result which adjusted the position of only a coil bobbin. It is the experimental result which adjusted the position of only the center core. FIG. 3 is an enlarged view of a main part of FIG. 2. It is a figure which shows the other structural example of a fixing unit. FIG. 10 is a diagram illustrating still another example of the structure of the fixing unit.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 includes a printer, a copier, a facsimile machine, and a multifunction machine having both functions of transferring a toner image onto the surface of a printing medium such as printing paper based on image information input from the outside. Or the like.

An image forming apparatus 1 shown in FIG. 1 is, for example, a tandem color printer. The image forming apparatus 1 includes a square box-shaped apparatus main body 2 that forms (prints) a color image on a sheet therein, and discharges the sheet on which the color image is printed on the upper surface of the apparatus main body 2. The discharge tray 3 is provided.
In the lower part of the apparatus main body 2, a paper feed cassette 5 for storing paper is disposed. In addition, a stack tray 6 for supplying manually fed sheets is disposed in the center of the apparatus main body 2. An image forming unit 7 is provided on the upper part of the apparatus main body 2. The image forming unit 7 forms an image on a sheet based on image data such as characters and designs transmitted from the outside of the apparatus.

  As shown in FIG. 1, a first transport path 9 for transporting paper fed from the paper feed cassette 5 to the image forming unit 7 is disposed on the left side of the apparatus main body 2. A second transport path 10 for transporting the sheet fed from the stack tray 6 to the image forming unit 7 is provided. A fixing unit (fixing device) 14 that performs a fixing process on a sheet on which an image is formed by the image forming unit 7 and a sheet on which the fixing process has been performed are disposed in the upper left part of the apparatus main body 2. And a third transport path 11 for transporting to the surface.

  The paper feed cassette 5 can be replenished by pulling it out of the apparatus main body 2 (for example, the front side in FIG. 1). The paper feed cassette 5 includes a storage unit 16 in which at least two types of paper having different sizes in the paper feed direction can be selectively stored. Note that the paper stored in the storage unit 16 is fed out to the first transport path 9 side by sheet by the paper feed roller 17 and the separating roller 18.

The stack tray 6 can be opened and closed on the outer surface of the apparatus main body 2, and manual sheets are loaded one by one or a plurality of sheets are stacked on the manual feed portion 19. Note that the sheets placed on the manual feed unit 19 are fed out one by one by the pickup roller 20 and the separating roller 21 to the second conveyance path 10 side.
The first transport path 9 and the second transport path 10 merge before the registration roller 22, and the paper supplied to the registration roller 22 waits here for a while, and after adjusting skew and timing, It is sent out toward the next transfer unit 23. The full color toner image on the intermediate transfer belt 40 is secondarily transferred to the sheet by the secondary transfer unit 23 on the sent sheet. Thereafter, the sheet on which the toner image is fixed by the fixing unit 14 is reversed in the fourth conveyance path 12 as necessary, and a full-color toner image is also formed on the surface opposite to the first by the secondary transfer unit 23. Secondary transferred. After the toner image on the opposite surface is fixed by the fixing unit 14, the toner image is discharged to the discharge tray 3 by the discharge roller 24 through the third conveyance path 11.

The image forming unit 7 includes four image forming units 26 to 29 that form black (B), yellow (Y), cyan (C), and magenta (M) toner images. The intermediate transfer unit 30 is configured to synthesize and carry the toner images of the respective colors formed in 29.
Each of the image forming units 26 to 29 includes a photosensitive drum 32, a charging unit 33 disposed so as to face the peripheral surface of the photosensitive drum 32, and a periphery of the photosensitive drum 32 on the downstream side of the charging unit 33. A laser scanning unit 34 for irradiating a laser beam to a specific position on the surface, and a developing unit disposed on the downstream side of the laser beam irradiation position from the laser scanning unit 34 and facing the peripheral surface of the photosensitive drum 32 35 and a cleaning unit 36 disposed on the downstream side of the developing unit 35 and facing the peripheral surface of the photosensitive drum 32.

The photosensitive drums 32 of the image forming units 26 to 29 are rotated counterclockwise in the drawing by a drive motor (not shown). Further, in the developing units 35 of the image forming units 26 to 29, black toner, yellow toner, cyan toner, and magenta toner are stored in the toner boxes 51, respectively.
The intermediate transfer unit 30 straddles the rear roller 38 disposed near the image forming unit 26, the front roller 39 disposed near the image forming unit 29, and the rear roller 38 and the front roller 39. The four intermediate transfer belts 40 and four transfer units arranged so as to be capable of being pressed against each other via the intermediate transfer belt 40 at positions downstream of the developing unit 35 on the photosensitive drums 32 of the image forming units 26 to 29. And a roller 41.

In the intermediate transfer unit 30, the toner images of the respective colors are superimposed and transferred onto the intermediate transfer belt 40 at the positions of the transfer rollers 41 of the image forming units 26 to 29. Become.
The first transport path 9 and the second transport path 10 are for transporting paper fed from the paper feed cassette 5 and the stack tray 6 to the intermediate transfer unit 30 side, and are located at predetermined positions in the apparatus main body 2. Are provided in front of the intermediate transfer unit 30 and a registration roller 22 for timing the image forming operation and the paper feeding operation in the image forming unit 7. .

  The fixing unit 14 performs processing for fixing the unfixed toner image on the paper by heating and pressurizing the paper on which the toner image has been transferred by the image forming unit 7. The fixing unit 14 includes a pair of rollers including, for example, a heating pressure roller (pressure member) 44 and a fixing roller 45, and the pressure roller 44 is made of, for example, metal, and the fixing roller 45 is made of a metal core. It has a surface layer (for example, silicon sponge) of a material and an elastic body and a release layer (for example, PFA). In addition, a heat roller (heating member) 46 is provided adjacent to the fixing roller 45, and a heating belt 48 is wound around the cylindrical heat roller 46 and the fixing roller 45. The detailed structure of the fixing unit 14 will be described later.

  When viewed in the sheet conveyance direction, conveyance paths 47 are provided on the upstream side and the downstream side of the fixing unit 14, and the sheet conveyed through the intermediate transfer unit 30 is pressurized through the upstream conveyance path 47. It is introduced into the nip between the roller 44 and the fixing roller 45. The paper that has passed between the pressure roller 44 and the fixing roller 45 is guided to the third conveyance path 11 through the conveyance path 47 on the downstream side.

The third transport path 11 transports the paper on which the fixing process has been performed by the fixing unit 14 to the discharge tray 3. For this reason, a transport roller 49 is disposed at an appropriate position in the third transport path 11, and the discharge roller 24 is disposed at the outlet thereof.
[Details of fixing unit]
Next, details of the fixing unit 14 applied to the image forming apparatus 1 of the present embodiment will be described.

  FIG. 2 is a longitudinal sectional view showing a structural example of the fixing unit 14. In FIG. 2, the orientation is turned counterclockwise by about 90 ° from the state in which the image forming apparatus 1 is mounted. Accordingly, the sheet conveying direction from the bottom to the top as viewed in FIG. 1 is from right to left as viewed in FIG. In addition, when the apparatus main body 2 is larger (multifunction machine etc.), it may be mounted in the direction shown in FIG. As another layout, the fixing unit 14 may be arranged in a posture inclined left or right from the state shown in FIG.

  The fixing unit 14 of this embodiment includes the pressure roller 44, the fixing roller 45, the heat roller 46, and the heating belt 48 as described above. The pressure roller 44 is formed, for example, by forming a Si rubber layer having a thickness of about 2 to 5 mm on a metal core (for example, SUS) having a diameter of about 50 mm and further forming a release layer (for example, FPA) on the surface layer thereof. ing. The fixing roller 45 has a silicon rubber sponge layer having a thickness of about 5 to 10 mm on a metal (for example, SUS) cored bar having a diameter of 45 mm, for example.

The heat roller 46 has a core metal made of a magnetic metal (eg, Fe) having a diameter of about 30 mm and a thickness of about 0.2 to 1.0 mm, and a release layer (eg, PFA) is formed on the surface thereof. Furthermore, the heating belt 48 is a ferromagnetic material (eg, Ni electroformed base material) having a base material thickness of, for example, 35 μm (1 μm = 1 × 10 ⁻⁶ m), and the surface layer has a thickness of about 200 to 500 μm. A thin elastic layer (for example, silicon rubber) is formed, and a release layer (for example, PFA) is formed on the outer surface thereof. In the case where the heating belt 48 does not have a heat generation function, a resin belt such as PI may be used.

As described above, since the fixing roller 45 has an elastic layer of silicon sponge as a surface layer, a flat nip is formed between the heating belt 48 and the fixing roller 45. A halogen heater 44 a is provided inside the pressure roller 44.
In addition, the fixing unit 14 includes an IH coil unit 50 outside the heat roller 46 and the heating belt 48 (not shown in FIG. 1). The IH coil unit 50 includes an induction heating coil 52, a pair of arch cores (first cores) 54, a pair of side cores (first cores) 56, and a center core (second core) 58.

〔coil〕
In the example of FIG. 2, the induction heating coil (coil) 52 is disposed on a virtual arc surface along the arc-shaped outer surface in order to perform induction heating in the arc-shaped portions of the heat roller 46 and the heating belt 48. . Actually, a coil bobbin (coil holding portion) 53, which will be described later with reference to FIG. 8, is disposed outside the heat roller 46 and the heating belt 48, and the induction heating coil 52 is disposed on the bobbin 53 in a winding shape. It is a configuration. The coil bobbin 53 is formed in a semi-cylindrical shape along the outer surface of the heat roller 46. The material of the bobbin 53 is preferably a heat-resistant resin (for example, PPS, PET, LCP), and the coil 52 is fixed using, for example, a silicon-based adhesive.

[First core]
As shown in FIG. 2, the center core 58 is located in the center, and the arch core 54 and the side core 56 are arranged so as to make a pair on both sides thereof. Of these, the arch cores 54 on both sides are ferrite cores formed in a cross-sectional arch shape that is symmetrical to each other, and the overall length is longer than the winding region of the induction heating coil 52. The side cores 56 on both sides are ferrite cores formed in a block shape. The side cores 56 on both sides are connected to one end (lower end in FIG. 2) of each arch core 54, and these side cores 56 cover the outside of the winding region of the induction heating coil 52.

  For example, the arch core 54 is arranged at a plurality of locations at intervals in the longitudinal direction of the heat roller 46 (FIG. 3). In the present embodiment, the width of the arch core 54 is about 10 mm. In addition, the higher the arrangement density of the arch cores 54, the better the magnetic field induction performance. However, even if the density is reduced to some extent, the performance does not decrease much, so that high cost performance can be obtained within a range where sufficient performance can be exhibited. It is preferable to set the arrangement density. Further, when adjusting the temperature distribution of the heating belt 48 in the axial direction, it is possible to cope with the problem by adjusting the arrangement density of the arch cores 54. In the present embodiment, for example, the arrangement density of the arch core 54 is set to about 1/2 to 1/3 as a whole, and the arrangement density at both ends of the induction heating coil 52 is set higher than that near the center, so that the end region We are also improving the temperature drop.

  Further, one side core 56 has a length of about 30 to 60 mm, and a plurality of side cores 56 are continuously arranged in the longitudinal direction of the heat roller 46 without any interval. The total length of the range in which the side core 56 is disposed corresponds to the length of the winding region of the induction heating coil 52. By arranging a plurality of side cores 56 in this way, there is an effect of leveling the fluctuation width of the temperature distribution due to the arrangement of the arch core 54. The arrangement of the cores 54 and 56 is determined, for example, according to the magnetic flux density (magnetic field strength) distribution of the induction heating coil 52, and the arch core 54 has been removed by a certain distance. The side core 56 compensates for the magnetic field focusing effect at the location, and leveles the magnetic flux density distribution (temperature difference) in the longitudinal direction.

For example, a resin core holder (not shown) is provided outside the arch core 54 and the side core 56, and the arch core 54 and the side core 56 are supported by the core holder. The material of the core holder is also preferably a heat resistant resin (for example, PPS, PET, LCP).
In the example of FIG. 2, a thermistor and a thermostat 62 are installed inside the heat roller 46. The thermistor or the like 62 can be disposed inside a portion of the heat roller 46 that generates a large amount of heat, particularly due to induction heating. More practically, a non-contact type sensor is disposed on the heating belt 48 below the coil unit 50 to detect the outer surface temperature of the belt 48.

[Second core]
The center core 58 shown in FIGS. 2 and 3 is, for example, a ferrite core having a cylindrical cross section (outer diameter is about 18 mm). In the center of the center core 58, a shaft member 59 of non-magnetic metal (SUS, etc.) or heat-resistant resin (PPS, PET, LCP, etc.) is inserted in the center in the axial direction. It has a length corresponding to a sheet passing width of 13 inches (for example, about 340 mm). When the paper is used, an alternating current of 20 kHz or more (alternating frequency is 30 kHz, for example) is used to avoid an audible area.

(Shielding member)
Further, a shielding member 60 is attached to the center core 58 along its outer surface. The shielding member 60 has a thin plate shape and is formed to be curved in an arc shape as a whole. For example, the shielding member 60 may be installed in a state where it is embedded in the thick portion of the center core 58 as shown in the figure, or may be installed in a state of being attached to the outer surface of the center core 58. The shielding member 60 can be attached using, for example, a silicon-based adhesive.

  In addition, as a structure of the shielding member 60, a nonmagnetic and highly conductive member is preferable, for example, oxygen-free copper is used. The shielding member 60 generates a reverse magnetic field by an induced current caused by the penetration of a magnetic field perpendicular to the surface thereof, and shields it by canceling the complex magnetic flux (perpendicular magnetic field). Further, by using a highly conductive member, Joule heat generation due to an induced current can be suppressed and a magnetic field can be efficiently shielded. In order to improve the conductivity, for example, methods such as (1) selecting a material with as low a specific resistance as possible and (2) increasing the thickness of the member are effective. Specifically, the plate thickness of the shielding member 60 is preferably 0.5 mm or more, and in this embodiment, for example, a thickness of 1 mm is used.

  As shown in FIG. 2, when the shielding member 60 is in a position (shielding position) close to the surface of the heating belt 48, the magnetic resistance increases around the induction heating coil 52 and the magnetic field strength decreases. On the other hand, when the center core 58 rotates 180 ° (the direction is not particularly limited) from the state shown in FIG. 2 and the shielding member 60 moves to a position (retracted position) farthest from the heating belt 48, the periphery of the induction heating coil 52 As a result, the magnetic resistance is reduced, a magnetic path is formed through the arch core 54 and the side core 56 on both sides centering on the center core 58, and a magnetic field acts on the heating belt 48 and the heat roller 46.

[Details of Center Core]
FIG. 3 is a plan view of the periphery of the center core 58. The center core 58 extends in the width direction of the sheet perpendicular to the sheet passing direction (the arrow direction in FIG. 3), and its total length is slightly larger than the maximum sheet passing width (for example, A3 length, A4 width).
The IH coil unit 50 is equipped with, for example, a stepping motor 66, and the shaft member 59 is rotated by the power of the motor 66. Therefore, a driven gear 59a is attached to one end portion of the shaft member 59, and the output gear 66a of the stepping motor 66 is engaged with the driven gear 59a. When the stepping motor 66 is driven, the shaft member 59 is rotated by the power, and the center core 58 can be integrally rotated around the longitudinal axis.

  At this time, in order to detect the rotation angle (rotational displacement amount from the reference position) of the center core 58, an index 72 is provided at one end of the shaft member 59, and a photo interrupter 74 is combined therewith. The position of the index 72 is set to a reference position related to the rotation angle of the center core 58, and the index 72 reacts (for example, shades) to the photo interrupter 74 at the reference position.

  The rotation angle of the center core 58 can be controlled by, for example, the number of drive pulses applied to the stepping motor 66, and a controller (magnetic shielding amount adjusting means) 83 is attached to the stepping motor 66. The control unit 83 can be configured by, for example, a control IC, an input / output driver, a semiconductor memory, and the like. The detection signal from the photo interrupter 74 is input to the control IC through the input driver, and based on this, the control IC can detect the reference position of the center core 58. On the other hand, the control IC is notified of information relating to the current paper size from an image formation control unit (not shown). In response to this, the control IC reads information on the rotation angle suitable for the paper size (angle when the reference position is 0 degree) from the semiconductor memory (ROM), and reaches the target rotation angle. Drive pulses are output at regular intervals. The drive pulse is applied to the stepping motor 66 through the output driver, and the stepping motor 66 operates in response to the drive pulse. The adjustment of the rotation angle of the center core 58 according to various paper sizes will be described later.

  In the example shown in FIG. 3, three types of the first shielding member 60 a, the second shielding member 60 b, and the third shielding member 60 c are used as the shielding member (reference numeral 60 in FIG. 2) in the axial direction (longitudinal direction) of the center core 58. ) Are divided and arranged. These first to third shielding members 60a, 60b, and 60c are different in arrangement and length in the axial direction of the center core 58, and also in length in the circumferential direction of the center core 58 (width covering the center core 58). ing. Hereinafter, this point will be described.

FIG. 4 is a diagram illustrating the axial arrangement of the first to third shielding members 60a, 60b, and 60c with respect to the center core 58, their lengths, and the circumferential width.
As shown in FIG. 4A, the three types of shielding members 60a, 60b, and 60c are provided symmetrically in the axial direction of the center core 58, and the first shielding member 60a is provided at both ends of the center core 58. The 2nd shielding member 60b and the 3rd shielding member 60c are arranged side by side in order toward the center from there. At this time, the third shielding member 60c located on the innermost side (close to the center) is provided outside the paper passing area W1 corresponding to the minimum paper size. The second shielding member 60b is provided outside the sheet passing area W2 corresponding to the intermediate sheet size, and the first shielding member 60a is provided outside the sheet passing area W3 larger by one size. ing. In this arrangement, for example, the maximum paper size is 13 inches (340 mm), and the smaller paper sizes are A3 (297 mm), A4 vertical (210 mm), and A5 vertical (149 mm), for a total of 4 It can correspond to various paper sizes. The axial lengths WP1, WP2, and WP3 of the shielding members 60a, 60b, and 60c are set in accordance with the corresponding paper sizes.

  In this embodiment, the boundary positions of the shielding members 60a, 60b, and 60c actually bite in (approach) up to about 10 ± 5 mm with respect to the respective paper passing areas W1, W2, and W3. It is set. As described above, the reason why the shielding members 60a, 60b, and 60c are set to bite into the paper passing areas W1, W2, and W3 is that the temperature in the non-paper passing area is usually higher than the temperature in the paper passing area. Therefore, in consideration of heat transfer from the non-sheet passing region, it is possible to easily balance the temperature distribution in the vicinity of the boundary by biting each shielding member 60a, 60b, 60c to the above extent. .

[Width of shielding member in circumferential direction]
FIGS. 4B and 4G: When the four types of paper sizes are supported as described above, the width of the first shielding member 60a viewed in the circumferential direction is set to 240 degrees at the center angle A1 of the center core 58. FIG. Yes.
FIGS. 4C and 4F: The width of the second shielding member 60b viewed in the circumferential direction is set to 160 degrees at the central angle A2 as shown in FIG.
4D and 4E: The width of the third shielding member 60c viewed in the circumferential direction is set to 80 degrees at the central angle A3.

FIG. 5 is a diagram illustrating an operation example associated with the rotation of the center core 58. In FIG. 5, illustration of the bobbin 53 is omitted for convenience of explanation. Moreover, although the 1st shielding member 60a is mentioned as an example, it is the same also about the other 2nd and 3rd shielding members 60b and 60c.
FIG. 5A shows an operation example when the first shielding members 60a at both ends are switched to the retracted positions as the center core 58 rotates. In this case, the magnetic field generated by the induction heating coil 52 passes through the heating belt 48 and the heat roller 46 through the side core 56, the arch core 54 and the center core 58. At this time, eddy currents are generated in the heating belt 48 and the heat roller 46, which are ferromagnetic materials, and Joule heat is generated due to the specific resistance of each material, and heating is performed.

  FIG. 5B: When the first shielding member 60a is switched to the shielding position, the first shielding member 60a is located on the magnetic path at the positions of both ends of the center core 58 (outside of the sheet passing area). The generation of is partially suppressed. As a result, the amount of heat generated is suppressed in the non-sheet passing area, and excessive heating of the heating belt 48 and the heat roller 46 can be prevented.

  Incidentally, the coil bobbin 53 shown in FIG. 2 has an envelope shape on the upper surface side on which the induction heating coil 52 is placed and the lower surface side facing the heat roller 46 (in the above example, the heating belt 48 corresponds). The center position for drawing the envelope circle and the rotation center position of the heat roller 46 are generally concentric as described in the prior art.

  If the thickness of the bobbin 53 is set to 2 mm, for example, the strength of the bobbin 53 is high and the molding becomes easy, and the heat roller 46 (in the above example, the heating belt 48 is applicable) from the lower surface side of the bobbin 53. ) Is set to 2 mm, for example, the heating belt 48 does not contact the lower surface side of the bobbin 53, and a clearance between the lower surface side of the bobbin 53 and the outer surface of the heat roller 46 can be secured.

  Further, according to the simulation results by the present inventors, if the coil 52 can be close to the heat roller 46, the heat generation performance is improved, the warm-up time of the fixing unit 14 is shortened, and the center core 58 can be close to the heat roller 46. It has been found that the magnetic shielding performance is improved and the temperature of the heat roller 46 in the non-sheet passing area can be suppressed.

  However, in order to satisfy both the above-described strength and formability of the bobbin 53 and the clearance between the bobbin 53 and the heat roller 46, the center core 58 and the heat roller 46 (in the above example, the heating belt 48 is applicable). Since the clearance needs to be about 4 mm, there is a concern that the center core 58 cannot be brought close to the heat roller 46.

One method for overcoming this concern is to drill a hole through the bobbin 53 along the longitudinal direction at the top of the bobbin 53 that faces the center core 58.
Therefore, first, using a coil bobbin having the hole (thickness is set to 2 mm, which is different from the coil bobbin 53 in FIG. 2 and not given a reference numeral), the position of the bobbin and the position of the center core 58 are variously changed. Thus, the optimum structure of the IH coil unit 50 is derived.

The evaluation for deriving this optimum structure is carried out for three items: (1) temperature distribution of the heat roller 46, (2) heat generation performance, and (3) magnetic shielding performance. Details will be described below.
FIG. 6 shows experimental results in which only the position of the coil bobbin having the hole is changed without changing the position of the center core 58.

  6A: Temperature distribution in the longitudinal direction of the heat roller 46 (corresponding to the heating belt 48 in the above example), and the surface temperature of the belt 48 measured immediately after the fixing unit 14 has been warmed up. It is. When the gap between the lower surface side of the bobbin and the belt 48 is 2 mm (indicated by a solid line in the figure), the temperature gradient is steep particularly at both end portions of the belt 48, whereas the gap is In the case of 5.5 mm (indicated by the alternate long and short dash line in the figure), the temperature gradient becomes gentle and the end temperature sag is worsening. That is, it is understood that it is preferable not to separate the bobbin from the belt 48 in view of the tendency of the end temperature sag.

  FIG. 6B shows the time required for warming up the fixing unit 14. If the gap between the bobbin and the belt 48 is changed in the range of 2 to 5.5 mm, this gap peaks in the vicinity of 3 to 4 mm, and particularly when the gap is 2 mm, the time required for warm-up becomes longer. ing. In other words, in view of this peak characteristic, it can be seen that when the gap between the lower surface side of the bobbin and the belt 48 is set to 3 to 4 mm, the position is optimum with respect to the flow of magnetic flux.

  Although not shown, when the gap between the bobbin and the belt 48 is changed in the range of 2 to 4.5 mm and the temperature of the heating belt 48 in the non-sheet passing region is measured, as the gap increases. The temperature was falling. However, since it can be assumed that this is affected by the edge temperature sagging, the magnetic shielding performance due to the change in the gap is considered to be equivalent.

Next, FIG. 7 shows an experimental result in which only the center core 58 is changed without changing the position of the coil bobbin having the hole.
FIG. 7A: Temperature distribution in the longitudinal direction of the belt 48. Similar to FIG. 6A, when the distance between the surface of the center core 58 and the belt 48 is 2 mm (indicated by a solid line in the figure). When the temperature gradient becomes steep and the distance is 5.5 mm (indicated by the alternate long and short dash line in the figure), the end temperature sag is getting worse. That is, also in this case, it is preferable that the center core 58 is not separated from the belt 48.

  FIG. 7B is a measurement of the temperature of the heating belt 48 in the non-sheet passing area. When the distance between the center core 58 and the belt 48 is changed within a range of 2 to 4.5 mm, the temperature of the sheet passing area (indicated by a circle in the figure) is first controlled by the temperature controller. On the other hand, the temperature in the non-sheet passing area (indicated by □ and Δ in the figure) is gradually increasing. It can be assumed that as the distance increases, the ratio of the magnetic flux that reaches the belt 48 directly without passing through the center core 58 increases, and the magnetic shielding performance is not the gap between the bobbin and the belt 48 described above. It can be seen that it depends greatly on the distance between the center core 58 and the belt 48, and it is preferable that the center core 58 is not separated from the belt 48.

Although not shown, the time required for warming up the fixing unit 14 was measured, but no clear tendency appeared. That is, it can be seen that the heat generation performance depends not on the distance between the center core 58 and the belt 48 but on the influence of the gap between the bobbin and the belt 48 described above.
[Summary]
From the above results, the heat generation performance depends on the position of the coil bobbin, and the magnetic shielding performance depends on the position of the center core, and the appropriate gap between the lower surface side of the bobbin and the surface of the belt 48 is 3 to 4 mm. Therefore, the center position of the envelope circle on the lower surface side of the bobbin is not concentric with the rotation center position of the heat roller 46, and the bobbin should be shifted upward away from the heat roller 46.

On the other hand, the distance between the center core 58 and the belt 48 is not separated even by 1 mm, and the position of the center core 58 is maintained or shifted downward toward the heat roller 46 so as to contact the envelope circle on the lower surface side of the bobbin, for example. It is better to configure.
By the way, the bobbin used in this experiment has a hole in the opposite portion of the center core 58, and this hole causes a decrease in the strength of the bobbin. In addition, since radiant heat from the heat roller 46 side is transmitted to the coil 52 through the hole, the coil 52 is adversely affected. On the other hand, cooling air from a fan that cools the coil 52 is transmitted through the hole to the heat roller 46. Can reach the roller 46 side and adversely affect the roller 46 side.

Therefore, the coil bobbin 53 of this embodiment closes the portion facing both the center core 58 and the belt 48.
More specifically, as shown in FIG. 8 in which FIG. 2 is enlarged, the coil bobbin 53 is configured by an envelope circular surface, has a curved surface portion 53b on which the coil 52 is placed, and has a thickness of 2 mm. Yes. The center position of the envelope circle is shifted upward by about 0.8 mm (indicated by a in the drawing) from the rotation center position of the heat roller 46.

As a result, the gap between the lower surface of the bobbin 53 and the surface of the belt 48 hardly changes in the vicinity of the side core 56, and about 3.3 mm can be secured in the vicinity of the center core 58, thereby satisfying the heat generation performance described above.
Furthermore, since the center core 58 has a surface in contact with the envelope circle on the lower surface side of the bobbin 53 and does not need to be shifted upward at least, the above-described magnetic shielding performance can be satisfied.

  The bobbin 53 can be moved upward, and the gap between the lower surface of the bobbin 53 and the surface of the belt 48 can be secured to about 3.3 mm in the vicinity of the center core 58. Accordingly, the curved surface portions 53b and 53b can be seen in FIG. A counter plate (opposite surface portion) 53 a that faces both the center core 58 and the belt 48 is formed therebetween. The opposing plate 53a is configured to be flat and thin (thickness b is 1 mm) as compared with the curved surface portion 53b, and is integrally formed of the same material as the curved surface portion 53b.

  Further, as a result of the gap between the lower surface of the bobbin 53 and the surface of the belt 48 being about 3.3 mm, as for the distance c from the surface of the center core 58 to the upper surface of the facing plate 53a, the shape accuracy of the bobbin 53, deformation, etc. The distance d from the lower surface of the plate 53a to the surface of the heating belt 48 is 1.5 mm which can also cope with the vibration of the belt 48. The thickness b of the plate 53a is 1 mm at which the moldability can be secured, and the linear distance L from the surface of the heating belt 48 facing the surface of the counter plate 53a to the surface of the center core 58 is b + c + d = 3.3 mm.

  By the way, the shift amount a of the bobbin 53 in the above embodiment is about 0.8 mm, and this is a gap of about 2.5 mm when the center position of the envelope circle and the rotation center of the heat roller are viewed concentrically. It was because it had established. In other words, when the concentric gap is set to about 2.0 mm, the shift amount a can be set to about 1.3 mm.

On the other hand, the above-described center core 58 is configured so that the surface thereof is in contact with the envelope circle on the lower surface side of the bobbin 53, but is not necessarily limited to this form.
Specifically, as shown in FIG. 9, if the linear distance f between the surface of the center core 58 and the envelope circle of the lower surface 53c indicated by a two-dot chain line in the drawing is 1 mm or less, the above-mentioned conditions are satisfied. Can still be met.

[Other examples of fixing unit structure]
Next, FIG. 10 is a diagram illustrating still another example of the structure of the fixing unit 14. In this structure example, a heating belt (heating member) 48 is wound around the outer periphery of a fixing roller (heating member) 45 having a heat insulating elastic layer, so that the heat capacity of the belt 48 can be reduced and the warm-up time is increased. Can be shortened more. Between the arch core 54 and the coil 52, a ring-shaped circuit unit (not shown) arranged along the winding of the coil 52 is arranged. The unit is provided with a comb-like open fixed ring circuit and a movable ring circuit in contact with the circuit to form a closed circuit, and the change in the paper size is accommodated by canceling out the magnetic field. Except for these configurations, the configuration of FIG. 10 is the same as that of FIG.

That is, also in the case of FIG. 10, the coil bobbin 53 can be shifted upward to form the facing plate 53 a between the core 58 and the belt 48. In addition to these structural examples, the present invention can also be applied to various fixing units of the outer packaging IH system.
As described above, according to the present invention, the center core 58 and the heat roller 46 (including the heating belt 48 in FIG. 2 and the fixing roller 45 and the heating belt 48 in FIG. 10) are desired to be close to each other. The main point between the heat roller 46 and the heat roller 46 is to be closed by the coil bobbin 53.

  According to the present embodiment, a method (outer packaging IH) in which the cylindrical heat roller 46 is induction-heated by the magnetic field generated by the coil 52 to heat and melt the toner image is adopted, and the arch core 54 and the side core 56 are In order to form a magnetic path for guiding the magnetic field generated by the coil 52, the coil 52 is disposed around the coil 52. As shown in FIG. 2, when the control unit 83 rotates the center core 53 and moves the shielding member 60 to the retracted position, the magnetic field generated by the coil 52 is guided to the cores 54, 56, and 58 to the heat roller 46. An eddy current is generated and magnetic induction heating is performed. On the other hand, when the control unit 83 rotates the center core 58 and moves the shielding member 60 to the shielding position, the magnetic resistance in the magnetic path increases, the magnetic field strength decreases, and the heat generation amount of the heat roller 46 is reduced.

  Here, as in the prior art, the center position of the envelope circle of the coil bobbin 53 and the rotation center position of the heat roller 46 are not concentric, but the center position of the envelope circle and the rotation center position are made different (FIG. 8). Is moved away from the heat roller 46 and closer to the center core 58 than in the prior art, and the center core 58 is not separated from the heat roller 46. As a result, the coil bobbin 53 can be disposed between the center core 58 and the heat roller 46 while securing the proximity state between the center core 58 and the heat roller 46, and heat generation performance, magnetic shielding performance, and arrangement space can be ensured. It becomes possible to achieve both.

  Further, the coil bobbin 53 is brought close to the center core 58. However, since the bobbin 53 is flat and the thin facing plate 53a faces the center core 58, the proximity state between the center core 58 and the heat roller 46 is maintained. While securing, the space between the center core 58 and the heat roller 46 can be reliably closed by the bobbin 53, that is, the counter plate 53a. As a result, the strength of the bobbin 53 is improved as compared with the case where the counter plate 53a is not provided, the coil 52 is not easily affected by the radiant heat from the heat roller 46 and the heating belt 48, and the heating belt 48 is further reduced. It becomes difficult to be affected by the cooling air.

  Furthermore, if the linear distance L from the surface of the heating belt 48 to the surface of the center core 58 is set within a range of 3.0 mm to 4.0 mm, the center core 58 and the heating belt 48 can be brought close to each other, and magnetic shielding performance can be secured. . Further, the counter plate 53a can be disposed between the center core 58 and the heating belt 48, and the distance between the coil 52 on the curved surface portion 53b and the heating belt 48 is improved, so that the heat generation performance can be reliably maintained.

  Further, in addition to the case where the surface of the center core 58 is in contact with a virtual envelope circle extending the curved surface portion 53b, as shown in FIG. The opposing plate 53 a can be disposed between the center core 58 and the heating belt 48 while ensuring a proximity state to the 48.

Furthermore, since the heat generation performance and the magnetic shielding performance are satisfied, a good toner image is formed, and as a result, the reliability of the image forming apparatus 1 is improved.
The present invention can be implemented with various modifications without being limited to the above-described embodiments. For example, the specific form of each part including the arch core 54 and the side core 56 is not limited to the illustrated one, and can be appropriately modified.

  And in any of these cases, as described above, both the performance and the arrangement can be satisfied.

1 Printer (image forming device)
7 Image forming unit 14 Fixing unit (fixing device)
44 Pressure roller (Pressure member)
45 Fixing roller (heating member)
46 Heat roller (heating member)
48 Heating belt (heating member)
50 IH coil unit 52 Induction heating coil (coil)
53 Coil bobbin (coil holder)
53a Opposite plate (opposite surface)
53b Curved surface portion 53c Lower surface 54 Arch core (first core)
56 Side core (first core)
58 Center core (second core)
60 shielding member 83 control unit (magnetic shielding amount adjusting means)

Claims (6)

Fixing for fixing the toner image on the sheet by at least heat from the heating member in the conveyance process by sandwiching the sheet on which the toner image is transferred in the image forming unit between the rotating heating member and the pressure member and conveying the sheet. A device,
A coil that is disposed along an outer surface of the heating member and generates a magnetic field for induction heating the heating member;
A coil holding portion having an envelope circular surface facing the outer surface of the heating member, and placing the coil;
A first core arranged on the opposite side of the heating member across the coil and made of a magnetic material to form a magnetic path around the coil;
A second core, which is provided between the first core and the heating member as viewed in the direction of generation of the magnetic field by the coil, and is made of a magnetic material so as to form a magnetic path together with the first core. When,
A shielding member that is provided along the outer surface of the second core and is made of a nonmagnetic metal so as to shield magnetism in a magnetic field generated by the coil;
And a magnetic shielding amount adjusting means for switching between a shielding position where the shielding member shields magnetism and a retreating position allowing passage of magnetism with the rotation of the second core,
The center position of the envelope circle of the coil holding part is different from the rotation center position of the heating member, and the position of the second core is maintained or brought close to the heating member, while the coil holding part is separated from the heating member. A fixing device, wherein the fixing device is disposed so as to be separated from and close to the second core. The fixing device according to claim 1,
The coil holding portion includes a curved surface portion formed of an envelope circular surface, and a facing surface portion that is formed flat and faces both the heating member and the second core. Fixing device. The fixing device according to claim 1,
The coil holding portion includes a curved surface portion configured by an envelope circular surface, a facing surface portion that is opposed to both the heating member and the second core, and is thinner than the curved surface portion. A fixing device. The fixing device according to any one of claims 1 to 3,
A heating belt is wound around the heating member, and a linear distance L (mm) from the surface of the heating belt facing the coil holding portion to the surface of the second core facing the coil holding portion; When
3.0 ≦ L ≦ 4.0
A fixing device characterized by satisfying the above. The fixing device according to claim 2 or 3,
When the linear distance f (mm) from the virtual envelope circle defined by the lower surface facing the heating member of the curved surface portion to the surface of the second core facing the facing surface portion,
0 ≦ f ≦ 1.0
A fixing device characterized by satisfying the above.   An image forming apparatus comprising: the fixing device according to claim 1 mounted on an image forming apparatus; and a toner image formed by an image forming unit using the fixing device is fixed on a sheet.

Images