JP5016497B2 - Image forming apparatus - Google Patents

Description

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

  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 unit and saving energy (for example, see Patent Document 1). In recent years, the electromagnetic induction heating method (IH) having the possibility of rapid heating and high-efficiency heating has been attracting attention. From the viewpoint of energy saving when fixing a color image, the electromagnetic induction heating is combined with the belt method. Many things 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 above-mentioned electromagnetic induction heating method, 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 is passed through the fixing unit. In particular, there are the following prior arts as size switching means in the outer package IH (see, for example, Patent Documents 2 and 3).

  In the first prior art (Patent Document 2), a magnetic member is divided into a plurality of pieces and arranged in the sheet passing width direction, and a part of the magnetic member is placed on the exciting coil in accordance with the sheet size (sheet passing width) to be passed. To be separated. In this case, it is considered that the heat generation efficiency is lowered by separating the magnetic member from the exciting coil in the non-sheet passing area, and the heat generation amount is smaller than the area corresponding to the sheet having the minimum sheet passing width.

In the second prior art (Patent Document 3), another conductive member is disposed outside the minimum sheet passing width inside the heat generating roller, and the position of the conductive member is switched within or outside the magnetic field range. It is. In this prior art, first, the conductive member is positioned outside the magnetic field range, and the heat generating roller is heated by electromagnetic induction. When the heat generating roller rises to near the Curie temperature due to the temperature rise, the conductive member is brought into the magnetic field range. By moving, the magnetic flux is leaked from the heat generating roller outside the minimum sheet passing width to prevent overheating.
JP-A-6-31801 Japanese Patent Laying-Open No. 2003-107941 (FIGS. 2 and 3) Japanese Patent No. 3527442 (FIG. 10)

  In the above prior art, in order to improve the productivity more than the current state, an excessive temperature rise suppression effect is required more than ever. That is, when a small size of paper is passed, even if the magnetic field is shielded outside the paper passing area, if a magnetic field (magnetic flux) leaks little by little from there, the leaked magnetic field causes a heating roller, etc. The temperature rises and eventually overheats. As a means for preventing this, for example, a method of making the area of the magnetic shielding plate larger than the current state is conceivable.

  However, if the area of the shielding plate is enlarged too much, this time, even when the shielding plate is retracted, the magnetic field is affected. For example, even if the shielding plate is retracted when it is necessary to perform induction heating with the full width of the heating roller, induction heating becomes insufficient if the magnetic field is blocked by the influence of the shielding plate even at the retracted position. not appropriate. Therefore, even if the excessive temperature rise suppression effect is improved as compared with the current situation, there is a limit to the expansion of the area of the shielding plate.

  Therefore, the present invention can obtain an effect of suppressing an excessive temperature rise in the end region of the heating roller or the like without excessively increasing the area of the shielding plate, and the influence of the shielding plate as much as possible when the shielding plate is retracted. The purpose is to provide technology that makes it difficult to be affected.

  According to the present invention, a sheet on which a toner image has been transferred by an image forming unit is transported by being sandwiched between a heating member and a pressure member, and in this transport process, the toner image is fixed on the sheet by at least heat from the heating member. An image forming apparatus including a fixing unit. In particular, the fixing unit is disposed along the outer surface of the heating member, is disposed on the opposite side of the heating member with a coil that generates a magnetic field for induction heating of the heating member, and is disposed around the coil. A core made of a magnetic material to form a path, and a magnetic shielding means for switching between a shielded state that shields magnetism generated by the coil in a non-sheet passing region of the heating member and an allowed state that allows passage of magnetism When the magnetic shield means is in an allowable state, it is installed in a ring shape between the coil and the heating member. A non-magnetic metal magnetic adjustment member that shields magnetism together with the shielding member by generating an electric current is provided.

  As described above, in the present invention, it is possible to cope with switching of the paper size by shielding the magnetism in the non-sheet passing region of the heating member by the magnetic shielding means or allowing the passage of magnetism. However, as described above, if the shielding member area is made extremely large aiming only at the magnetic shielding effect, the influence on the magnetic field increases when the magnetism is passed (when the member is retracted). It is difficult to achieve a complete magnetic shielding effect with only the magnetic shielding means.

  Therefore, the present invention increases the suppression effect of excessive temperature rise without increasing the shielding member area by arranging a ring-shaped magnetic adjustment member between the coil and the heating member, and also allows the passage of magnetism. It makes it difficult to influence the magnetic field when allowed. That is, the magnetic adjustment member supplements the magnetic shielding by the magnetic shielding means, and prevents the magnetic flux leaking from the shielding by the magnetic shielding means in the non-sheet passing region from acting on the heating member.

  Furthermore, the magnetic adjustment member employed in the present invention has the following merits. That is, since the magnetic adjustment member has a ring shape, when a magnetic field perpendicular to the inner surface of the ring (complex magnetic flux) penetrates, an induced current is generated in the circumferential direction of the ring, and the direction opposite to the penetrating magnetic field is generated therefrom. Generates a demagnetizing field. By canceling the magnetic field (complex magnetic flux) that the demagnetizing field penetrates the inside of the ring in the vertical direction, the magnetic adjustment member can supplement the magnetic shielding. On the other hand, when the magnetic field passes through the inside of the ring in both directions or passes in a U-turn, no induced current is generated and the magnetic shielding effect is not exhibited.

  The inventors of the present invention have found the optimal arrangement of the magnetic adjustment member between the coil and the heating member, paying attention to the properties of the magnetic adjustment member as described above. When the magnetic shielding means is in a shielded state, the magnetic adjustment member in the optimal arrangement is such that a magnetic field (magnetic flux) leaking from the magnetic shielding member tries to penetrate the inside of the ring, so that an induced current is generated in the ring to generate a magnetic field. Can be canceled. On the other hand, when the magnetic shielding means is in an allowable state, the magnetic field generated around the coil passes in the bidirectional or U-turn direction inside the ring, so that almost no induced current is generated in the ring. The passage of the magnetic field can be allowed. Thereby, the excessive temperature rise suppression effect can be demonstrated more than the present condition, without enlarging the area of the member for shielding extremely.

  The magnetic shielding means has a shielding member made of a non-magnetic metal. The shielding member shields the magnetism in a magnetic path passing through the core and the heating member around the coil, and magnetically shields the outside of the magnetic path. It may be configured to switch to a retreat position that allows the passage of.

  In this case, even if the area of the shielding member is not increased so much, magnetic shielding can be supplemented by the magnetic adjustment member in a state where it is switched to the shielding position. On the other hand, in a state where the shielding member is switched to the retracted position, the magnetic adjustment member does not affect the magnetic field, so that the heating member can be sufficiently induction-heated, which can contribute to shortening the warm-up time. .

  When the heating member is a ferromagnetic material, the magnetic shielding means is provided between the core and the heating member in addition to the shielding member as seen in the direction of magnetic field generation by the coil, and the shielding member is provided along the outer surface. The movable movable core can be configured. The movable core itself can move (rotate) to switch the shielding member between the shielding position and the retracted position.

  Further, when the heating member is a non-magnetic material, the magnetic shielding means is a movable type provided inside the heating member to form a magnetic path penetrating the heating member and provided with a shielding member along the outer surface. It is the structure which has the core of. In this case, since the heating member penetrates the magnetic field, a shielding member may be disposed inside the heating member.

  In the present invention, the magnetic shielding means may have the following configuration. In other words, the magnetic shielding means comprises a heating member made of a magnetic shunt metal material, so that when the heating member is heated to a Curie temperature or higher, it shields the magnetism within the magnetic field generated by the coil, but in a state where the Curie temperature is not reached. The heating member is induction-heated by passing magnetism.

  In this case, the heating member itself can shield or pass magnetism by temperature (Curie temperature) without particularly switching the shielding member between the shielding position and the retracted position. Even in this case, it is possible to pass magnetism without affecting the magnetic field during heating while assisting the magnetic shielding effect by optimally arranging the magnetic adjustment member.

  In addition to the above, the present invention has the following features.

(1) The magnetic adjustment member is made of a nonmagnetic metal made of copper, and preferably has a thickness in the range of 0.5 mm to 3 mm. In other words, the magnetic adjusting member needs to reduce the specific resistance (electric resistance) of the member as much as possible in order to effectively suppress the Joule heat generation and shield the magnetism. With the above-mentioned member dimensions, it is possible to secure good conductivity by sufficiently reducing the specific resistance of the magnetic adjustment member, obtain a sufficient magnetic shielding effect, and reduce the weight of the magnetic adjustment member. Can do.

(2) The magnetic adjustment member has an angular ring shape having a common outer peripheral portion, and the inside thereof is divided into a plurality of parts as viewed in the longitudinal direction of the heating member.
(3) Alternatively, the magnetic adjustment member may be composed of a plurality of rectangular ring-shaped members arranged in the longitudinal direction of the heating member.

  As described above, the magnetic adjustment member exhibits a magnetic shielding effect within the inner range of the ring. Therefore, the magnetic adjustment member is divided into a plurality of ring portions as described in (2) above, or ( By arranging and arranging a plurality of members as in 3), it is possible to deal with paper of various sizes.

  The present invention can improve the excessive temperature rise suppression effect of the heating member without increasing the area of the shielding member as compared with the current state. Further, when the magnetic shielding is not performed, since the shielding member does not affect the magnetic field, the warm-up time is not hindered.

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. Paper discharge section (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. A stack tray 6 for supplying manually fed sheets is disposed at 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, from the right to the left. Is provided with a second transport path 10 for transporting paper fed from the stack tray 6 to the image forming section 7. In the upper left part of the apparatus main body 2, a fixing unit 14 that performs a fixing process on a sheet on which an image is formed by the image forming unit 7 and a sheet that has been subjected to the fixing process are conveyed to the sheet discharging unit 3. 3 conveyance paths 11 are arranged.

  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 passes through the third conveyance path 11 and is discharged to the paper discharge unit 3 by the discharge roller 24.

  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 includes a rear roller (drive roller) 38 disposed near the image forming unit 26, a front roller (driven roller) 39 disposed near the image forming unit 29, and a rear roller. 38 and the intermediate transfer belt 40 disposed between the front roller 39 and the downstream side of the developing unit 35 in the photosensitive drum 32 of each of the image forming units 26 to 29 can be pressed through the intermediate transfer belt 40. The four transfer rollers 41 are provided.

  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 transports the paper fed from the paper feed cassette 5 to the intermediate transfer unit 30 side, and includes a plurality of transport rollers 43 disposed at predetermined positions in the apparatus main body 2. And a registration roller 22 disposed in front of the intermediate transfer unit 30 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 roller pair including, for example, a heating pressure roller 44 and a fixing roller 45. Of these, the pressure roller 44 is made of, for example, metal, and the fixing roller 45 is made of a metal core and an elastic body. It has a surface layer (for example, silicon sponge) and a release layer (for example, PFA). A heat roller 46 is provided adjacent to the fixing roller 45, and a heating belt 48 is wound around the 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 paper discharge unit 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 (first 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.

  The fixing unit 14 includes the pressure roller 44, the fixing roller 45, the heat roller 46, and the heating belt 48 as described above. While the pressure roller 44 is made of metal, the fixing roller 45 has an elastic layer of silicon sponge as a surface layer. Therefore, 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. The base material of the heating belt 48 is a ferromagnetic material (for example, Ni), a thin elastic layer (for example, silicon rubber) is formed on the surface layer, and a release layer (for example, PFA) is formed on the outer surface thereof. The core of the heat roller 46 is a magnetic metal (for example, Fe), and a release layer (for example, PFA) is formed on the surface thereof.

  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 54, and a pair of side cores 56 and a center core 58.

〔coil〕
In the example of FIG. 2, the induction heating coil 52 is disposed on a virtual arcuate surface along the arcuate outer surface in order to perform induction heating in the arcuate portions of the heat roller 46 and the heating belt 48. Actually, for example, a resin cover (not shown) is arranged outside the heat roller 46 and the heating belt 48, and the induction heating coil 52 is arranged in a winding shape on the resin cover.

〔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 and the side core 56 are arranged at a plurality of positions at intervals in the longitudinal direction of the heat roller 46. The arrangement of the cores 54 and 56 is determined in accordance with, for example, the magnetic flux density (magnetic field strength) distribution of the induction heating coil 52.

  In the example of FIG. 2, a thermistor 62 (which may be a thermostat) is installed inside the heat roller 46. The thermistor 62 can be disposed inside a portion of the heat roller 46 that generates a large amount of heat, particularly due to induction heating.

[Center Core]
The center core 58 is a ferrite core having a cylindrical cross section, for example. As with the heat roller 46, the center core 58 has a length corresponding to the maximum sheet passing width of the sheet. Although not shown in FIG. 2, the center core 58 is connected to a rotation mechanism (not shown), and can be rotated around the longitudinal axis by the rotation mechanism. The rotation mechanism will be further described later.

(Shielding member)
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, 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.

[Magnetic adjustment member]
In addition, the IH coil unit 50 is provided with magnetic adjustment members 90 on both sides centered on the center core 58 and positioned between the induction heating coil 52 and the arch core 54. As seen in the cross section in FIG. 2, each magnetic adjustment member 90 is provided at a position overlapping the winding region of the induction heating coil 52. Each of these magnetic adjustment members 90 is a nonmagnetic metal (for example, oxygen-free copper) having a rectangular ring shape in plan view. Since it is ring-shaped, the inside of the magnetic adjustment member 90 is hollow (hollow), and the cross-sectional shape is shown only at both ends in FIG. However, as shown in FIG. 2, the magnetic adjustment member 90 is formed to be curved in an arc shape as a whole. For example, the center of curvature of the magnetic adjustment member 90 substantially coincides with the center of rotation of the heat roller 46, and the radius of curvature is smaller than the virtual arc surface on which the induction heating coil 52 is disposed.

  FIG. 3 is a perspective view showing a structural example (1) of the magnetic adjustment member 90. As described above, the magnetic adjustment member 90 has a rectangular ring shape as a whole, and its four sides are constituted by a pair of linear portions 90a opposed in the width direction and a pair of arc portions 90b opposed in the longitudinal direction. Although only one magnetic adjustment member 90 is shown in FIG. 3, the IH coil unit 50 is provided with, for example, four magnetic adjustment members 90 in total, two at each end portion in the longitudinal direction of the heating belt 48. It has been. Note that the magnetic adjustment member 90 is attached (fixed) to the inner surface of the resin cover (in which the induction heating coil 52 is arranged in a winding shape on the upper side).

  FIG. 4 is a perspective view showing a structural example (2) of the magnetic adjustment member 90. In this structural example (2), a flange portion 90c is formed on the pair of arc portions 90b. Thereby, the cross section of the magnetic adjustment member 90 can be enlarged in the circumferential direction, the overall rigidity can be increased, and the electrical resistance can be further reduced. In addition, the flange part may be formed on a pair of linear part 90a.

[Characteristics of magnetic adjustment member]
FIG. 5 is a conceptual diagram for explaining the characteristics of the magnetic adjustment member 90. In FIG. 5, the magnetic adjustment member 90 is simplified as a simple wire model.

  5A: When a penetrating magnetic field (complex magnetic flux) is generated in the ring surface (virtual plane) in the vertical direction (one direction) with respect to the ring-shaped magnetic adjustment member 90, the magnetic adjustment member is thereby generated. An induced current is generated in 90 circumferential directions. Then, since a magnetic field (demagnetizing field) opposite to the penetrating magnetic field is generated by electromagnetic induction, they cancel each other and cancel the magnetic field. In the present embodiment, when the shielding member 60 is moved to the shielding position, the magnetic shielding effect is supplemented by using this magnetic field canceling effect.

  In FIG. 5, (B): As shown in the upper stage, a penetrating magnetic field is generated bi-directionally on the ring surface of the ring-shaped magnetic adjustment member 90. The case of (± 0) is assumed. In this case, almost no induced current is generated in the magnetic adjustment member 90. Therefore, the magnetic adjustment member 90 hardly exhibits the magnetic field canceling effect, and the bidirectional magnetic field passes through the magnetic adjustment member 90. The same applies to the case where the magnetic field passes in the U-turn direction inside the magnetic adjustment member 90 as shown in the lower part. In the present embodiment, when the shielding member 60 is moved to the retracted position, the magnetic adjustment member 90 is fixedly arranged at a position where the balance of the magnetic field passing through the ring of the magnetic adjustment member 90 is balanced, thereby affecting the magnetic field. Is suppressed.

  FIG. 5C shows a case where a magnetic field (complex magnetic flux) is generated substantially in parallel with the ring surface of the ring-shaped magnetic adjustment member 90. In this case as well, almost no induced current is generated in the magnetic adjustment member 90, and therefore no magnetic field canceling effect is generated. This pattern is not adopted in this embodiment.

  The inventors of the present invention pay attention to the fact that both the effect of shielding the magnetism and the effect of not shielding the magnetism are obtained by the states (A) and (B) in FIG. By fixing and arranging the shield member 60, the magnetic shield effect by the shield member 60 is assisted, and the magnetic field is prevented from being affected when the shield member 60 is retracted. First, a magnetic shielding method using the shielding member 60 will be described.

[Magnetic shielding means]
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.

  FIG. 6 is a diagram illustrating the arrangement of the shielding member 60 and the magnetic adjustment member 90 as an example. 6A corresponds to the retracted position, and FIG. 6B corresponds to the shielding position. 6A and 6B are a side view and a bottom view of the center core 58 and the magnetic adjustment member 90, respectively. In the figure, the outer surface of the center core 58 is shaded.

  As described above, the center core 58 has a total length substantially equal to or longer than the maximum sheet passing width W3 of the sheet. At this time, the shielding member 60 is divided into two in the longitudinal direction of the center core 58, and these are symmetrical to each other. Each shielding member 60 has a triangular shape in a plan view or a bottom view, for example, and a portion corresponding to the apex of the triangle is positioned closer to the center of the center core 58. That is, at the position closer to the center of the center core 58, the length of the shielding member 60 in the circumferential direction is the shortest, and from there, the shielding member 60 is gradually expanded in the circumferential direction toward both side ends of the center core 58. ing.

  Further, the shielding member 60 is provided on both outer sides of the minimum sheet passing width W1 orthogonal to the sheet passing direction, and the shielding member 60 is provided only slightly in the range of the minimum sheet passing width W1. The shielding member 60 reaches slightly outside the maximum sheet passing width W2 at both ends of the center core 58. The minimum sheet passing width W1 and the maximum sheet passing width W2 are determined by the minimum or maximum size sheet that can be printed by the image forming apparatus 1.

  Further, in the present embodiment, the ratio of the length of the shielding member 60 to the outer peripheral length of the center core 58 in the rotational direction is different in the axial direction (longitudinal direction) of the center core 58. At this time, when the ratio of the length (Lc) of the shielding member 60 to the outer peripheral length (L) of the center core 58 is defined as the coverage (= Lc / L), the coverage is small inside the center core 58, and from there the axial direction It becomes larger toward the outside (both ends). Specifically, the coverage is minimum in the vicinity of the minimum sheet passing region (the range of the minimum sheet passing width W1), and conversely, is maximum at both ends of the center core 58.

  Correspondence to the paper size (paper passing width) is realized by switching the position of the shielding member 60 and partially suppressing the generated magnetic flux. At this time, the rotation angle (rotational displacement amount) of the center core 58 is varied according to the paper size (paper passing width), and the magnetic shielding amount is reduced as the paper size increases, and conversely, the shielding amount increases as the paper size decreases. By doing so, it is possible to prevent both ends of the heat roller 46 and the heating belt 48 from overheating. In FIG. 6, only the rotation in the counterclockwise direction is indicated by an arrow, but the center core 58 may be rotated in the clockwise direction. Further, the paper passing direction may be opposite to the direction shown in FIG.

[Division of magnetic adjustment member]
Moreover, in the example shown in FIG. 6, the magnetic adjustment member 90 is one rectangular ring shape as a whole, and the inside is divided | segmented into several (divided) by the circular arc part 90b. For this reason, the magnetic adjustment member 90 has a structure that is divided into a plurality of ring portions when viewed in the longitudinal direction of the heating belt 48. As described above, if a single ring is divided into a plurality of parts, it is possible to cope with different sheet passing widths W1, W2, and W3 depending on the sheet size. For example, when the sheet size is the minimum (minimum sheet passing width W1), the magnetic shielding effect can be supplemented by the entire three ring portions of each magnetic adjustment member 90. Further, when the paper size is in the range from the minimum to the middle (minimum sheet passing width W1 to intermediate sheet passing width W2), the magnetic shielding effect can be supplemented by the two ring portions from the outside of each magnetic adjustment member 90. When the sheet size is the maximum (maximum sheet passing width W3), no induction current is generated in all three ring portions of each magnetic adjustment member 90, and the influence on the magnetic field generated by the induction heating coil 52 can be eliminated. it can.

[Another form of magnetic adjustment member]
FIG. 7 is a diagram showing an example of the arrangement of another embodiment of the magnetic adjustment member 90. In the example shown in FIG. 7, a plurality of magnetic adjustment members 90 are individually arranged independently. That is, the individual magnetic adjustment members 90 are one ring each and are not electrically connected to each other. In this case as well, the paper passing widths W1, W2, and W3 that differ depending on the paper size can be handled. For example, when the sheet size is minimum (minimum sheet passing width W1), the magnetic shielding effect can be compensated for by all the magnetic adjustment members 90. Further, when the paper size is in the range from the minimum to the middle (minimum sheet passing width W1 to intermediate sheet passing width W2), two magnetic adjustment members 90 are provided from each outside (total of eight) to provide a magnetic shielding effect. Can make up. When the sheet size is the maximum (maximum sheet passing width W3), no induction current is generated in all the magnetic adjustment members 90, and the influence on the magnetic field generated by the induction heating coil 52 can be eliminated.

[Rotation mechanism of magnetic shielding means]
Next, a mechanism for rotating the center core 58 about the axis, that is, a mechanism for displacing the shielding member 60 between the shielding position and the retracted position will be described. FIG. 8 is a side view showing the configuration of the rotation mechanism 64 of the center core 58 and a partial cross-sectional view (longitudinal cross-section along the line BB) showing the operation thereof.

  In FIG. 8, (A): The rotation mechanism 64, for example, decelerates the rotation of the stepping motor 66 by the reduction mechanism 68 and drives the drive shaft 70 to rotate the center core 58. For example, a worm gear is used as the speed reduction mechanism 68, but other mechanisms may be used. Further, in order to detect the rotation angle of the center core 58 (rotational displacement from the reference position), a disk 72 with a slit is provided at the end of the drive shaft 70, and a photo interrupter 74 is combined therewith.

  8B: The drive shaft 70 is connected to one end of the center core 58, and supports the center core 58 without penetrating through the center core 58. 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 control circuit (not shown) is attached to the rotation mechanism 64. The control circuit can be constituted 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 detects the current rotation angle (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 from the semiconductor memory (ROM), and outputs drive pulses for reaching the target rotation angle at a constant cycle. 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.

  FIG. 9 is a diagram illustrating an operation example associated with the rotation of the center core 58. Each will be described below.

[Permitted state]
FIG. 9A shows an operation example when the shielding member 60 is switched to the retracted position 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.

[Function of magnetic adjustment member (1)]
At this time, inside the magnetic path passing through the heating belt 48 and the heat roller 46 through the side core 56, the arch core 54 and the center core 58, for example, the magnetic flux leaking from the arch core 54 passes through the inside of the ring of the magnetic adjustment member 90, and the heating belt 48. And passes through the heat roller 46 and passes through the inside of the ring of the magnetic adjustment member 90 and converges on the arch core 54 again. Such leaked magnetic flux does not pass through the center core 58 and the side core 56, but contributes to induction heating of the heating belt 48 and the heat roller 46 inside the magnetic path.

  With respect to such a leaked magnetic flux, the magnetic adjustment member 90 is in the state shown in the lower part of FIG. In other words, since the magnetic field passes in the U-turn direction inside the ring of the magnetic adjustment member 90, the magnetic adjustment member 90 allows the leakage magnetic flux to pass without exerting a canceling effect. Thereby, it can contribute to shortening of warm-up time, without inhibiting the induction heating of the heating belt 48.

[Shielded state]
(B) in FIG. 9: Next, an operation example when the shielding member 60 is switched to the shielding position is shown. In this case, since the shielding member 60 is positioned on the magnetic path outside the minimum sheet passing area, the generation of the magnetic field is partially suppressed. As a result, the amount of heat generation is suppressed outside the minimum sheet passing area, and excessive heating of the heating belt 48 and the heat roller 46 can be prevented. Further, the amount of shielding of the magnetic field can be adjusted by changing the rotation angle of the center core 58 little by little. For example, when the rotation angle of the center core 58 is increased counterclockwise from the position (B) in FIG. 9, shielding is not performed on the left side in the figure and a magnetic field is generated, but on the right side in the figure The magnetic field is subsequently shielded. In this case, since the magnetic field intensity generated as a whole is lower than the position (A) in FIG. 9, the amount of heat generation can be reduced accordingly.

[Function of magnetic adjustment member (2)]
At this time, the magnetic path passing through the heating belt 48 and the heat roller 46 through the side core 56, the arch core 54, and the center core 58 is shielded. It passes through the inside and tries to pass through the heating belt 48 and the heat roller 46. If the heating belt 48 and the heat roller 46 are induction-heated by such leaked magnetic flux, the shielding effect by the shielding member 60 is lowered.

  In the present embodiment, the magnetic adjustment member 90 is in the state shown in FIG. 5A with respect to the magnetic flux that is about to leak from the arch core 54 in the shielded state. That is, since the leakage magnetic flux becomes an interlaced magnetic flux inside the ring of the magnetic adjustment member 90, the magnetic adjustment member 90 can exert a canceling effect on the leakage magnetic flux and can suppress the passage thereof. Thereby, since the magnetic adjustment member 90 can supplement the shielding effect by the shielding member 60, a sufficient magnetic shielding effect can be obtained without extremely increasing the area of the shielding member 60, and the heating belt 48. Further, the excessive temperature rise of the heat roller 46 can be suppressed as compared with the current situation.

〔Condition setting〕
Next, the condition setting found by the inventors of the present invention will be described with an example. First, when the magnetic adjustment member 90 supplements the magnetic shielding effect by the shielding member 60 as described above, it is preferable to use a non-magnetic and highly conductive member in order to suppress Joule heat generation due to the induced current and effectively shield the magnetism. In the present embodiment, oxygen-free copper or the like is used as the material as described above. In addition, in order to improve the conductivity of the magnetic adjustment member 90, it is necessary to select a material having as small a specific resistance as possible and to increase the thickness of the material (symbol t in the figure). Under the conditions found by the inventors of the present invention, the thickness t of the magnetic adjustment member 90 is preferably set within a range of 0.5 mm to 3 mm.

  FIG. 10 is a diagram showing various conditions set in the present embodiment. Regarding the angle at which the magnetic adjustment member 90 is installed with respect to the heating belt 48 and the heat roller 46 (symbol α in the figure), the inventors of the present invention present the following conditions. For example, the horizontal line (this is the horizontal line in FIG. 10 and different from the mounted state) passing through the center of the shaft center of the heat roller 46 is set as the reference angle (= 0 °). When the angle (deg) of the ring center L of the magnetic adjustment member 90 is taken in the counterclockwise direction from the center, the optimum condition for the angle α and the ring width WR for installing the magnetic adjustment member 90 is as follows. Can be set.

  FIG. 11 is a distribution curve representing the intensity distribution of the radial magnetic field viewed around the heat roller 46 (360 °). In FIG. 11, the horizontal axis represents the angle (deg) in the counterclockwise direction from the reference angle (= 0 °), and the vertical axis represents, for example, the radial magnetic field (A / m). A curve indicated by a thick solid line in FIG. 11 indicates a distribution when the shielding member 60 does not shield the magnetism (retracted position). A curve indicated by a dotted line in FIG. 11 indicates a distribution when the shielding member 60 performs magnetic shielding (shielding position). At this time, the magnetic adjustment member 90 is preferably arranged so as not to affect the magnetic field when the magnetic shielding is not performed.

  Therefore, assuming that the shielding member 60 is displaced to the retracted position and the magnetic field canceling effect does not work (the distribution of the thick solid line in the figure), the radial magnetic field is in the vicinity of 0 °, 90 °, and 180 °. Each has a peak of intensity, and it can be seen that almost no magnetic field is generated around 270 ° (directly below) where the induction heating coil 52 is not disposed.

  In such a magnetic field distribution, first, a point P at which the radial magnetic field becomes 0 on the distribution curve is taken, and values obtained by integrating the distribution curves in the 0 ° direction and 90 ° direction from this point P are the same (area S1 in the figure). = [Alpha] 1, [alpha] 2 is obtained as S2). If the magnetic adjustment member 90 is arranged within the range of the calculated angles α1 to α2, the magnetic field balance by the magnetic adjustment member 90 becomes 0 when the magnetic shielding is not performed, and as a result, the magnetic field is not disturbed. .

Then, an angle (= (α1 + α2) / 2) that is the midpoint between the angles α1 and α2 on the horizontal axis is obtained, and this is set as the angle of the ring center L at the retracted position. When the distance from the center of the heat roller 46 to the magnetic adjustment member 90 is a radius r, the length of a virtual arc located within the range of the angles α1 to α2 at the radius r is the optimum of the magnetic adjustment member 90. Ring width. In this example, the ring width WR can be obtained by the following equation. The radius r is larger than the radius of the heat roller 46 (D / 2 in the figure).
WR = r × {(α2−α1) / 180} × π

  As described above, the optimum condition can be set by setting the ring width of the magnetic adjustment member 90 to WR (the above equation) and the angle of the ring center to be installed to (α1 + α2) / 2. In particular, when the shielding member 60 is switched to the retracted position, the magnetic field balance (areas S1 and S2) is exactly 0 inside the ring, so that the generation of the magnetic field by the induction heating coil 52 is not hindered.

  Assuming that the above-described structural example is the first example of the fixing unit 14, the following structural examples (second to fifth examples) can be given in the present embodiment. Hereinafter, each structural example will be described. In any case, the same reference numerals as those in the first example are assigned to the configurations common to those in the first example, and redundant description thereof will be omitted. Even if the reference numerals are common, a description to that effect is added particularly when the materials are different.

[Second example]
FIG. 12 is a diagram illustrating a structural example (second example) of the fixing unit 14. In the second example, the toner image is fixed by the fixing roller 45 and the pressure roller 44 without using the heating belt. For example, a magnetic material similar to that of the above-described heating belt is wound around the outer periphery of the fixing roller 45, and the magnetic material is induction-heated by the induction heating coil 52. In this case, the thermistor 62 is provided outside the fixing roller 45 at a position facing the magnetic layer. The rest is the same as above, and the center core 58 can be rotated to adjust the shielding amount of the magnetic field. The magnetic adjustment member 90 is disposed between the induction heating coil 52 and the fixing roller 45.

[Third example]
FIG. 13 is a longitudinal sectional view showing a structural example (third example) of the fixing unit 14. The third example is different from the first example in that the heat roller 46 is made of a nonmagnetic metal (for example, SUS: stainless steel) and the center core 58 is disposed inside the heat roller 46. . In addition, an arch core 54 is connected at the center, and an intermediate core 55 is installed at the lower part thereof. The magnetic adjustment member 90 is disposed between the induction heating coil 52 and the heating belt 48.

  When the heat roller 46 is made of a nonmagnetic metal, the magnetic field generated by the induction heating coil 52 passes through the side core 56, the arch core 54, and the intermediate core 55, passes through the heat roller 46, and reaches the internal center core 58. The heating belt 48 is induction-heated by a penetrating magnetic field.

  In the case of the third example, as shown in FIG. 13, the state in which the shielding member 60 is separated from the intermediate core 55 is the retracted position. In this case, the magnetic shielding effect does not work and heating is performed in the maximum sheet passing region. The belt 48 is heated by induction. On the other hand, when the shielding member 60 is switched to a position facing the intermediate core 55 (shielding position), the magnetism is shielded, and excessive temperature rise is suppressed outside the sheet passing area.

[Fourth example]
FIG. 14 is a longitudinal sectional view showing a structural example (fourth example) of the fixing unit 14. In the fourth example, the heat roller 46 is made of a magnetic shunt metal (for example, an iron-nickel alloy), and is different from the first to third examples in that the magnetism is shielded by the temperature change of the heat roller 46 itself. Yes. That is, in the fourth example, when the heat roller 46 is heated to a temperature equal to or higher than the Curie temperature, the magnetism is lost and the magnetic shielding effect is exerted, so that an excessive temperature rise of the heat roller 46 itself can be prevented. On the other hand, while the heat roller 46 is heated within the range of the Curie temperature or less, the magnetism is passed by the magnetism of the magnetic shunt metal, so that the toner image can be fixed by Joule heat generation of the heat roller 46. The magnetic adjustment member 90 is disposed between the induction heating coil 52 and the heating belt 48.

  In the case of the fourth example, since it is not necessary to provide a shielding member, there is no need for a center core for installing the shielding member, and the structure can be simplified in that a rotating mechanism is not required.

[Fifth example]
FIG. 15 is a diagram illustrating a structural example (fifth example) of the fixing unit 14. In the fifth example, induction heating is performed not at the arcuate position of the heating belt 48 but at a planar position between the heat roller 46 and the fixing roller 45. In this case as well, the shielding amount of the magnetic field can be adjusted by rotating the center core 58. The magnetic adjustment member 90 is a flat ring and is installed between the induction heating coil 52 and the heating belt 48.

  The present invention can be implemented with various modifications without being limited to the above-described embodiments. For example, the cross-sectional shape of the center core 58 is not limited to a cylinder or a column, but may be a polygonal shape. Further, the shape of the shielding member 60 in plan view is not limited to a triangular shape, and may be a trapezoidal shape.

  Further, the shape and size of the ring of the magnetic adjustment member 90 and the number of parts to be divided are only examples, and are not particularly limited to the embodiment.

  In addition, 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.

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 (first example) of the fixing unit. It is a perspective view which shows the structural example (1) of a magnetic adjustment member. It is a perspective view which shows the structural example (2) of a magnetic adjustment member. It is a conceptual diagram for demonstrating the characteristic of a magnetic adjustment member. It is the figure which showed arrangement | positioning of a shielding member and a magnetic adjustment member as an example. It is the figure which showed arrangement | positioning of the magnetic adjustment member of another form as an example. It is the side view and partial sectional view which show the structure of a rotation mechanism. It is a figure which shows the operation example accompanying rotation of a center core. It is a figure which shows the various conditions set in one Embodiment. It is a distribution curve showing the intensity distribution of the radial direction magnetic field seen around the heat roller (360 °). FIG. 6 is a diagram illustrating a structural example (second example) of a fixing unit. FIG. 10 is a diagram illustrating a structural example (third example) of the fixing unit. FIG. 10 is a diagram illustrating a structural example (fourth example) of the fixing unit. FIG. 10 is a diagram illustrating a structural example (fifth example) of the fixing unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 14 Fixing unit 50 IH coil unit 52 Induction heating coil 54 Arch core 56 Side core 58 Center core 60 Shield member 62 Thermistor 64 Rotation mechanism 66 Stepping motor 68 Deceleration mechanism 90 Magnetic adjustment member

Claims (8)

A fixing unit that fixes the toner image onto the sheet by at least heat from the heating member is conveyed by sandwiching the sheet on which the toner image is transferred in the image forming unit between the heating member and the pressure member. An image forming apparatus comprising:
The fixing unit includes:
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 core disposed on the opposite side of the heating member across the coil, and a core made of a magnetic material to form a magnetic path around the coil;
Magnetic shielding means for switching between a shielding state that shields the magnetism generated by the coil in a non-sheet-passing region of the heating member and an allowable state that allows passage of magnetism;
When the magnetic shielding means is installed in a ring shape between the coil and the heating member and the magnetic shielding means is in the allowable state, an induction current due to a magnetic field is not generated, while the magnetic shielding means is in the shielding state. An image forming apparatus comprising: a nonmagnetic metal magnetic adjustment member that generates an induced current and shields the magnetism together with the shielding member. The image forming apparatus according to claim 1,
The magnetic shielding means includes
A shielding member made of a non-magnetic metal, the shielding member shielding a magnetism in a magnetic path passing through the core and the heating member around the coil, and a magnetic member outside the magnetic path; An image forming apparatus that switches to a retracted position that allows passage. The image forming apparatus according to claim 1, wherein
The heating member is ferromagnetic;
The magnetic shielding means includes
In addition to the shielding member, the movable member has a movable core provided between the core and the heating member as seen in the direction of generation of the magnetic field by the coil and provided with the shielding member along the outer surface. An image forming apparatus. The image forming apparatus according to claim 1, wherein
The heating member is a non-magnetic material;
The magnetic shielding means includes
An image forming apparatus comprising: a movable core provided inside the heating member so as to form a magnetic path penetrating the heating member and provided with the shielding member along an outer surface. The image forming apparatus according to claim 1,
The magnetic shielding means includes
By constituting the heating member from a magnetic shunt metal material, when the heating member is heated above the Curie temperature, the magnetism is shielded in the magnetic field generated by the coil, but the magnetism is passed when the Curie temperature is not reached. An image forming apparatus characterized in that the heating member is induction-heated. The image forming apparatus according to claim 1,
The magnetic adjustment member is
An image forming apparatus comprising a nonmagnetic metal made of copper and having a thickness in a range of 0.5 mm to 3 mm. The image forming apparatus according to any one of claims 1 to 6,
The magnetic adjustment member is
An image forming apparatus, characterized in that it has a single square ring shape with a common outer peripheral portion, and the inside thereof is divided into a plurality of parts as viewed in the longitudinal direction of the heating member. The image forming apparatus according to any one of claims 1 to 6,
The magnetic adjustment member is
2. An image forming apparatus comprising: a plurality of square ring-shaped members arranged in a longitudinal direction of the heating member.

Images