CN101482727A - Image forming apparatus - Google Patents

Abstract

A fixing unit (14) of an image forming apparatus (1) includes a heating member (48) having a first area where a sheet does not come into contact with the heating member (48) and a second area where the sheet comes into contact with the heating member (48). The fixing unit (14) further includes a coil (52) forming a magnetic field, cores (54,56) forming a magnetic path near the coil (52), a nonmagnetic magnetism adjusting member (90) arranged on the magnetic path and having a closed frame, and a switcher (58,60) capable of switching the magnetism adjusting member (90) between a first state where the magnetism adjusting member (90) generates an induction current resulting from the magnetic field to shield the magnetism in the first area, and a second state where the magnetism adjusting member (90) generates no induction current and the magnetism is not shielded in the first area.

Description

Image forming apparatus with a toner supply device

Technical Field

The present invention relates to an image forming apparatus including a fixing unit that passes a sheet bearing a toner image between a pair of heating rollers or a nip between a heating belt and a roller, and heats and melts toner that is not fixed to the sheet to fix the toner to the sheet.

Background

In recent years, from the viewpoint of shortening the warm-up time of the fixing unit, saving energy, and the like, the use of a belt system capable of reducing the heat capacity in an image forming apparatus has been attracting attention (see, for example, japanese patent laid-open No. 6-318001). In recent years, attention has been paid to an electromagnetic induction heating system (IH) capable of heating quickly and efficiently, and there are many products combining the electromagnetic induction heating system with a belt system in view of energy saving in fixing color images. When the belt system and the electromagnetic induction heating system are combined, an electromagnetic induction device is often disposed outside the belt (so-called "external IH") because of advantages such as easy arrangement and cooling of the coil and direct heating of the belt.

In the electromagnetic induction heating method, various techniques have been developed in order to prevent an excessive temperature rise in the sheet non-passage area in consideration of the width of the sheet passing through the fixing unit (sheet passing width). In particular, as a dimension switching device in the external IH, there are the following conventional techniques (for example, japanese patent laid-open publication No. 2003-107941 and japanese patent laid-open publication No. 3527442).

A first prior art (japanese patent laid-open publication No. 2003-107941 (fig. 2 and 3)) discloses: the magnetic member is divided into a plurality of pieces, arranged in the paper passage width direction, and a part of the magnetic member is disengaged from the exciting coil in accordance with the size of the paper (paper passage width) to be passed. In this case, in the paper non-passage area, the magnetic member is separated from the exciting coil, so that the heat generation efficiency is reduced, and the amount of heat generation is smaller than that in the area corresponding to the paper having the smallest paper passage width.

The second prior art (japanese patent publication No. 3527442 (fig. 10)) discloses: inside the heat generating roller, other conductive members are arranged outside the minimum paper passing width, and the position of the conductive member is switched within or outside the magnetic field range. In the second conventional technique, first, the conductive member is positioned outside the magnetic field range, and the heat generating roller is heated by electromagnetic induction. When the temperature of the heat generating roller rises to the vicinity of the Curie temperature, the conductive member is moved into the magnetic field range. Thus, the magnetic flux leaks from the heat generating roller outside the minimum paper passing width, and the paper non-passing area can be prevented from being excessively heated.

In the first and second prior arts, in order to further improve the operation efficiency as compared with the conventional case, it is necessary to obtain a higher effect of suppressing the excessive temperature rise. That is, in the case of passing small-sized paper, even if the magnetic field is shielded outside the paper passing region, if the magnetic field (magnetic flux) leaks from there little by little, the temperature of the heating roller or the like is raised by the leaked magnetic field, and in the near future, the temperature is excessively raised. As a method for preventing this, for example, a method of increasing the area of the magnetic shield panel more than the conventional one can be adopted.

However, if the area of the shield plate is made too large, the magnetic field is affected even if the shield plate is in the retracted state. For example, when it is necessary to inductively heat the entire width of the heating roller, even if the shield plate is retracted, if the shield plate blocks the magnetic field at the retracted position due to the influence of the shield plate, induction heating cannot be sufficiently performed, which is not preferable. Therefore, there is a limit to increase the area of the shield plate even if the effect of suppressing the excessive temperature rise is higher than the current one.

Disclosure of Invention

Therefore, an object of the present invention is to provide a technique for obtaining an effect of suppressing an excessive temperature rise in an end region of a heating roller or the like without making an area of a shield plate or the like excessively large, and for being not affected by the shield plate or the like as much as possible in a retracted state of the shield plate or the like.

In order to achieve the object, an image forming apparatus of the present invention includes: an image forming unit for transferring a toner image onto a sheet; and a fixing unit including a heating member and a pressing member, and configured to convey the sheet while sandwiching the sheet between the heating member and the pressing member, and fix the toner image on the sheet by using at least heat from the heating member during the conveyance; wherein the heating member is set to a first area and a second area according to a size of the sheet, the first area being an area where the sheet conveyed by the fixing unit does not contact when passing, and the second area being an area where the sheet conveyed by the fixing unit contacts when passing, the fixing unit further includes: a coil that generates magnetic gas for inductively heating the heating member to form a magnetic field; a magnetic core made of a magnetic material for forming a magnetic path in the vicinity of the coil; a magnetic adjustment member which is an annular non-magnetic metal and is disposed on the magnetic path; and a switching unit that can switch between a first state in which the magnetic adjustment member generates an induced current due to the magnetic field and performs magnetic shielding in the first region, and a second state in which the magnetic adjustment member does not generate the induced current and does not perform magnetic shielding in the first region.

In the image forming apparatus of the present invention, when the magnetic adjustment member is switched to the first state by the switching portion, the first region is magnetically shielded, and therefore, the first region can be prevented from being excessively heated. Further, when the magnetic adjustment member is switched to the second state by the switching portion, since the magnetic shielding is not performed in the first region, the induction heating of the first region can be promoted.

In the image forming apparatus of the above configuration, the coil is disposed along an outer surface of the heating member; the magnetic core is disposed at a position opposite to the heating member with the coil interposed therebetween; the magnetic adjustment member is disposed between the coil and the heating member; the switching unit is a magnetic shielding unit that can switch between a shielding state in which magnetic shielding is performed and an allowing state in which magnetic flux is allowed to pass through in the first region; the magnetic adjustment member is switched to the first state when the magnetic shield portion is switched to the shielding state, and the magnetic adjustment member is switched to the second state when the magnetic shield portion is switched to the permission state.

With this structure, the first region of the heating member is magnetically shielded by the magnetic shielding portion, or magnetic flux is allowed to pass, whereby switching of paper size can be accommodated. However, if the area of the magnetic shield portion is increased by focusing only on the effect of the magnetic shield, the magnetic flux is blocked and the magnetic field is greatly influenced when the magnetic flux is allowed to pass. Therefore, it is difficult to obtain a perfect magnetic shielding effect only with the magnetic shielding portion.

In the above configuration, the annular magnetic adjusting member is disposed between the coil and the heating member, so that the effect of suppressing the excessive temperature rise can be improved without increasing the area of the magnetic shielding portion, and the magnetic field is less likely to be affected when the magnetic flux is allowed to pass. That is, the magnetic adjusting member compensates for the magnetic shielding by the magnetic shielding portion, and the magnetic flux leaking when the first region magnetic shielding portion shields can be prevented from acting on the heating member.

Further, the magnetism adjustment member employed in the structure has the following advantages. That is, since the magnetic adjustment member is formed in a ring shape, if a vertical magnetic field (interlinkage magnetic flux) penetrates the inner side surface of the ring, an induced current is generated in the circumferential direction of the ring, and therefore, a reverse magnetic field in the opposite direction to the penetrating magnetic field is generated. Since the reverse magnetic field cancels the magnetic field (interlinkage magnetic flux) penetrating the inner side of the ring in the vertical direction, the magnetic adjusting member can compensate for the magnetic shielding. On the other hand, when the magnetic field passes back and forth through the inner side of the ring in both directions or passes through the inner side of the ring in the U-turn direction, no induced current is generated, and the effect of magnetic shielding is not exhibited.

The inventors of the present invention have focused on the properties of the magnetism adjustment member as described above, and found an optimum position of the magnetism adjustment member between the coil and the heating member. When the magnetic adjustment member is located at the optimum position, if the magnetic shielding portion is in the shielding state, the leaked magnetic field (magnetic flux) penetrates the inside of the ring, so that an induced current is generated in the ring, and the magnetic field can be canceled. On the other hand, when the magnetic shield portion is in the permission state, the magnetic field generated around the coil passes through the inside of the ring in both directions or in the U-turn direction, so that an induced current is hardly generated in the ring and the passage of the magnetic field can be permitted. Thus, the effect of suppressing the excessive temperature rise can be exhibited more effectively than the current situation without making the area of the shield member very large.

Preferred in the structure are: the magnetic shield portion has a shield member made of a non-magnetic metal, and the shield member is switched between a shield position for shielding the magnetic flux passing through the magnetic core and the heating member around the coil and a retracted position for allowing the magnetic flux to pass through the magnetic core.

With this configuration, even if the area of the shielding member is not made too large, the magnetic shielding can be compensated by the magnetism adjusting member in the state where the shielding member is switched to the shielding position. On the other hand, in the state where the shield member is switched to the retracted position, since the magnetic field is not affected by the magnetic adjustment member, the heating member can be sufficiently induction-heated, contributing to shortening the warm-up time thereof.

In the structure, the heating member may be constituted by a ferromagnetic body. In this case, it is preferable that: the magnetic shielding portion has a movable magnetic core in addition to the shielding member, is provided between the magnetic core and the heating member in a direction of a magnetic field generated by the coil, and is provided with the shielding member along an outer surface of the movable magnetic core.

In the above configuration, the heating member may be made of a non-magnetic material. In this case, it is preferable that: the magnetic shielding part has a movable magnetic core, is provided inside the heating member, forms a magnetic path penetrating the heating member, and is provided with the shielding member along an outer surface of the movable magnetic core. Since the magnetic field penetrates the heating member, the shielding member may be disposed inside the heating member.

Further, in the structure, the magnetic shield portion may be the heating member composed of a magnetic metal material. In this case, when the heating member is heated to the curie temperature or higher, the heating member is magnetically shielded in the magnetic field generated by the coil, and the heating member is inductively heated by passing the magnetic field in a state where the curie temperature is not reached. Thus, even if the shielding member is not switched to the shielding position or the retracted position, the heating member itself can magnetically shield or pass magnetism by temperature (curie temperature). In this configuration, the magnetic adjustment member is optimally arranged, so that the magnetic shielding effect can be compensated, and the magnetic flux can be passed without affecting the magnetic field during heating.

In the above-described configuration, the magnetic adjustment member may be a square ring shape having a common outer peripheral portion, and an inner portion of the square ring shape may be divided into a plurality of portions in a direction perpendicular to the sheet conveying direction. Alternatively, the magnetic adjustment member may include a plurality of square ring members aligned in a direction perpendicular to the sheet conveying direction.

Since the magnetic adjustment member exerts a magnetic shielding effect in the inner region of the ring as described above, the magnetic adjustment member can be divided into a plurality of ring portions or a plurality of members are arranged to accommodate paper of various sizes.

In the image forming apparatus of the above configuration, particularly in the fixing unit of another configuration, it is preferable that the coil is disposed along an outer surface of the heating member, is formed around a predetermined winding center, and generates the magnetic field; the magnetic core is disposed at a position opposite to the heating member with the coil interposed therebetween, and the magnetic path is formed around the coil through the center of the winding; the magnetic adjustment member has a prescribed ring center; the switching unit switches a state of the magnetic adjustment member between a first state and a second state, the first state being a state in which the magnetic adjustment member is shielded by substantially overlapping the ring center and the winding center on one axis in a middle of a magnetic path reaching the heating member through the winding center; the second state is a state in which the magnetism adjustment member is moved away from the magnetic circuit so that the ring center and the winding center are located on different axes, thereby allowing the passage of magnetism.

In this structure, with the use of the characteristics of the magnetic adjustment member, when the magnetic adjustment member is in the first state, a sufficient magnetic shielding effect can be obtained in the first region, and when the magnetic adjustment member is in the second state, the generation of the magnetic shielding effect can be suppressed in the first region. Therefore, the temperature can be sufficiently raised without lowering the heat generation efficiency at the time of induction heating the heating member, and the warm-up time can be shortened. In the second state, since the magnetic adjustment member can also overlap the winding region of the coil, it is not necessary to secure a housing space outside the coil and the core, and thus space can be saved. Further, since the magnetic adjustment member is annular, hollowed out in the middle, even if a sufficiently wide range (ring width) is secured, the weight of the member can be reduced. Therefore, the material cost can be reduced, and the power (e.g., motor output) at the time of changing the position of the magnetic adjustment member can be controlled to the minimum.

In the fixing unit having the above-described configuration, the coil and the core may be disposed outside the heating member (so-called outer package IH), or the coil and the core may be disposed inside the heating member (so-called inner package IH). In this case it is preferred that: the coil is disposed along an inner surface of a heating region in a circumferential direction of the heating member, is formed around a predetermined winding center, and generates a magnetic field for inductively heating the heating member in the heating region; the magnetic core is disposed inside the heating member together with the coil, and forms a magnetic path passing through the winding center of the coil; the magnetic adjustment member has a prescribed ring center; the switching unit switches a state of the magnetic adjustment member between a first state and a second state, the first state being a state in which the magnetic field is shielded by substantially overlapping the ring center and the winding center on one axis in a middle of a magnetic path reaching the heating member through the winding center; the second state is a state in which the magnetism adjustment member is moved away from the magnetic circuit so that the ring center and the winding center are located on different axes, thereby allowing the passage of magnetism.

In the above-described internal IH structure as well, by positioning the magnetism adjusting member at the shielding position inside the heating member, a sufficient magnetic shielding effect can be obtained by the magnetism adjusting member, and in the second state, since the magnetism adjusting member smoothly passes the magnetism, sufficient heating efficiency can be exhibited. In this case, the magnetic adjustment member, the coil, and the magnetic core can be disposed inside the heating member, and therefore, a sufficient space can be saved.

In the structure of the outsourced IH, it is preferable that: the switching portion enables the magnetic adjustment member to switch positions between the coil and the core. Further, in the structure including the IH, it is preferable that: the switching unit can switch the position of the magnetic adjustment member between the heating member and the coil. Both structures are such that in the first state, the magnetism-adjusting member can shield magnetism midway through the magnetic path through the center of the coil, and in the second state, the magnetism can be reliably allowed to pass.

In the structure of the external IH and the internal IH, it is preferable that: the switching unit sets a state in which a current flowing in a circumferential direction of the magnetism adjustment member when the coil generates the magnetic field is substantially 0 as the second state. That is, in the second state, if the current flowing through the magnetism adjustment member (in the loop) is 0, the magnetic field generated to the coil does not generate a reverse magnetic field, and therefore magnetic induction to the heating member is not hindered.

In addition, in the structure of the external IH and the internal IH, it is preferable that: the switching portion reciprocates the magnetic adjustment member between the first state and the second state, and is capable of adjusting a magnetic shielding amount in accordance with a displacement amount therebetween. With this configuration, when the switching unit switches the magnetic adjusting member to the position, the magnetic shielding is not substantially (completely) performed in the second state, and the amount of magnetic shielding can be gradually increased as the switching unit switches from the second state to the first state.

In the structure of the external IH and the internal IH, it is preferable that: the magnetic adjustment member is in a square ring shape, is divided into a plurality of square rings in a direction perpendicular to a sheet conveying direction, and has different ring widths in the sheet conveying direction, and is set to be the smallest in a vicinity of a minimum sheet passing area which is an area where a sheet of the smallest size among the sheets conveyed by the fixing unit contacts when passing.

Since the ring-shaped magnetic adjustment member exerts a magnetic shielding effect in the range inside the ring, the ring width is large, the amount of magnetic shielding is also large, the ring width is small, and the amount of magnetic shielding is also small in the plurality of magnetic adjustment members. Therefore, the loop width is set to be minimum in the vicinity of the minimum paper passage area where excessive temperature rise of the heating member is not likely to occur in general, and the loop width is increased as it is separated from the vicinity of the minimum paper passage area in the paper passage width direction (width direction of the heating member).

Preferred in the structure are: the magnetic adjustment member is made of a non-magnetic metal conductor made of a copper material, and has a width dimension of 1mm to 5mm and a thickness of 0.5mm to 3 mm. In order to effectively shield the magnetic field of the magnetic adjustment member by suppressing joule heat, the intrinsic resistance of the member needs to be reduced as much as possible. If the material is selected and sized as described above, the intrinsic resistance of the magnetism adjustment member can be made sufficiently small. As a result, good electrical conductivity can be ensured, a sufficient magnetic shielding effect can be obtained, and the weight of the magnetic adjustment member can be reduced.

In the present embodiment, the heating member may be a metal roll or a metal belt, and any of these methods is suitable for induction heating using a coil.

Drawings

FIG. 1 is a schematic view showing a configuration of an image forming apparatus according to an embodiment.

Fig. 2 is a longitudinal sectional view showing the fixing unit of the first embodiment.

Fig. 3 is a perspective view showing a structural example (1) of the magnetic adjustment member.

Fig. 4 is a perspective view showing a structural example (2) of the magnetic adjustment member.

Fig. 5A to 5C are schematic diagrams for explaining the characteristics of the magnetic adjustment member.

Fig. 6A and 6B are diagrams showing examples of the arrangement of the shield member and the magnetism adjustment member.

Fig. 7 is a diagram showing an example of the arrangement of the magnetism adjustment member according to another embodiment.

Fig. 8A and 8B are a side view and a partial sectional view showing the structure of the rotating mechanism.

Fig. 9A and 9B are diagrams showing an example of the operation accompanying the rotation of the center core.

Fig. 10 is a diagram showing various conditions set in one embodiment.

FIG. 11 is a graph showing a distribution of a radial magnetic field intensity distribution around a heat roller (360).

Fig. 12 is a diagram showing a first modification of the fixing unit according to the first embodiment.

Fig. 13 is a diagram showing a second modification of the fixing unit according to the first embodiment.

Fig. 14 is a diagram showing a third modification of the fixing unit according to the first embodiment.

Fig. 15 is a diagram showing a fourth modification of the fixing unit according to the first embodiment.

Fig. 16 is a longitudinal sectional view showing the fixing unit according to the second embodiment.

Fig. 17 is a perspective view showing a configuration example (1) of a magnetism adjustment member according to a second embodiment.

Fig. 18 is a perspective view showing a configuration example (2) of a magnetism adjustment member according to the second embodiment.

Fig. 19A and 19B are diagrams illustrating a method of performing magnetic adjustment by a magnetic adjustment member.

Fig. 20 is a distribution graph showing a radial magnetic field intensity distribution around the fixing roller (360 °).

Fig. 21A to 21C are plan views showing a plurality of arrangement examples of the magnetism adjustment member.

Fig. 22A and 22B are vertical sectional views showing modifications of the fixing unit according to the second embodiment.

Fig. 23 is a vertical cross-sectional view showing another modification of the fixing unit according to the second embodiment.

Fig. 24A and 24B are longitudinal sectional views showing a fixing unit according to a third embodiment.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a schematic diagram showing a configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 may be a printer, a copier, a facsimile machine, a complex machine having these functions, or the like, and performs printing by transferring a toner image onto a surface of a printing medium such as a printing sheet based on image information input from the outside.

The image forming apparatus 1 shown in fig. 1 is a tandem color printer. The image forming apparatus 1 has a rectangular box-shaped apparatus main body 2, and forms (prints) a color image on a sheet (sheet) in the apparatus main body 2. A paper discharge unit (paper discharge tray) 3 for discharging paper on which a color image is printed is provided on the upper surface of the apparatus main body 2.

In the apparatus main body 2, a paper feed cassette 5 for storing paper is disposed at a lower portion, a stacker tray 6 for manually feeding paper is disposed at a central portion, and an image forming portion 7 is disposed at an upper portion. The image forming unit 7 forms (transfers) a toner image on a sheet based on image data such as characters and patterns transmitted from the outside of the apparatus.

In fig. 1, a first conveyance path 9 for conveying the sheet fed from the sheet cassette 5 to the image forming unit 7 is disposed on the left side of the apparatus main body 2; a second conveyance path 10 for conveying the sheet drawn from the sheet stacking tray 6 to the image forming unit 7 is disposed from a right portion to a left portion of the apparatus main body 2. Further, at an upper left portion in the apparatus main body 2, there are disposed: a fixing unit 14 that performs a fixing process on the paper on which the image is formed in the image forming unit 7; and a third conveyance path 11 for conveying the paper subjected to the fixing process to the paper discharge unit 3.

The sheet cassette 5 can be replenished with sheets in a state where it is pulled out to the outside of the apparatus main body 2 (for example, to the front side in fig. 1). The paper feed cassette 5 has a housing portion 16, and paper sheets having different sizes in at least two kinds of paper feed directions can be selectively loaded in the housing portion 16. The sheets loaded in the storage section 16 are fed out one by one toward the first conveyance path 9 by the sheet feeding roller 17 and the distribution roller 18.

The stack tray 6 can be opened or closed outside the apparatus main body 2, and sheets can be placed one by hand or a plurality of sheets can be stacked on the manual sheet feeding section 19. The sheets set in the manual sheet feeding section 19 are fed out one by one to the second conveyance path 10 by the pickup roller 20 and the distribution roller 21.

The first conveyance path 9 and the second conveyance path 10 merge near the registration roller 22. The sheet fed to the registration roller 22 is temporarily on standby, and after the skew adjustment and timing adjustment, the sheet is sent to the second transfer unit 23. In the secondary transfer section 23, the full-color toner image on the intermediate transfer belt 40 is transferred to the fed paper (secondary transfer). After that, the toner image on the sheet is fixed in the fixing unit 14, the sheet is reversed in the fourth conveyance path 12 as necessary, and the full-color toner image is transferred by the second transfer portion 23 also on the surface opposite to the first surface (second transfer). Then, after the toner image on the opposite side is fixed in the fixing unit 14, the sheet is discharged to the sheet discharge portion 3 through the third conveyance path 11 by the discharge roller 24.

The image forming portion 7 has four image forming units 26, 27, 28, and 29, and forms toner images of respective colors of black (B), yellow (Y), cyan (C), and magenta (M). The image forming section 7 further includes an intermediate transfer section 30 for combining and carrying the toner images of the respective colors formed by the image forming units 26 to 29.

Each of the image forming units 26 to 29 includes: a photosensitive drum 32; a charging section 33 disposed opposite to the circumferential surface of the photoconductive drum 32; a laser scanning unit 34 for irradiating a laser beam to a specific position on the circumferential surface of the photoconductive drum 32 on the downstream side of the charging section 33; a developing unit 35 disposed to face the circumferential surface of the photoconductive drum 32 at a position downstream of the laser beam irradiation position of the laser scanning unit 34; and a cleaning portion 36 disposed to face the circumferential surface of the photosensitive drum 32 at a position on the downstream side of the developing portion 35.

The photosensitive drums 32 of the respective image forming units 26 to 29 are rotated counterclockwise in the drawing by a drive motor not shown. In the developing section 35 of each of the image forming units 26 to 29, a black toner, a yellow toner, a cyan toner, and a magenta toner are contained in each of the toner cartridges 51.

The intermediate transfer section 30 includes: a drive roller 38 disposed at a position near the image forming unit 26; a driven roller 39 disposed at a position near the image forming unit 29; an intermediate transfer belt 40 wound around the driving roller 38 and the driven roller 39; and four transfer rollers 41 corresponding to the photosensitive drums 32 of the image forming units 26 to 29. The transfer rollers 41 are disposed on the downstream side of the developing units 35 in the image forming units 26 to 29, and are capable of pressure-contacting the photosensitive drums 32 via the intermediate transfer belt 40.

In the intermediate transfer section 30, the toner images of the respective colors are transferred onto the intermediate transfer belt 40 in a superimposed manner at the positions of the transfer rollers 41 of the respective image forming units 26 to 29. As a result, a full-color toner image is finally formed on the intermediate transfer belt 40.

The first conveyance path 9 conveys the sheet drawn from the sheet feed cassette 5 to the intermediate transfer section 30. In the first conveyance path 9: a plurality of conveying rollers 43 disposed at predetermined positions in the apparatus main body 2; and a registration roller 22 disposed in the vicinity of the intermediate transfer section 30 and used for adjusting the timing of the image forming operation and the paper feeding operation of the image forming section 7.

The fixing unit 14 performs a process of fixing an unfixed toner image on the sheet by applying heat and pressure to the sheet to which the toner image is transferred in the image forming portion 7. The fixing unit 14 has a roller pair composed of a pressure roller 44 (pressure member) of a heating type and a fixing roller 45. The pressure roller 44 is a metal roller, and the fixing roller 45 has a metal core, an elastic surface layer (e.g., silicone sponge), and a release layer (e.g., PFA). A heat roller 46 is provided adjacent to the fixing roller 45, and a heating belt 48 (heating member) is wound around the heat roller 46 and the fixing roller 45. The detailed structure of the fixing unit 14 will be described later.

Conveyance paths 47 are provided upstream and downstream of the fixing unit 14 in the paper conveyance direction. The sheet conveyed through the intermediate transfer section 30 passes through the upstream conveyance path 47 and is introduced into a nip between the pressure roller 44 and the fixing roller 45 (heating belt 48). The sheet passing 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 conveyance path 11 conveys the paper on which the fixing process is performed in the fixing unit 14 to the paper discharge portion 3. To this end, a conveying roller 49 is provided at an appropriate position in the third conveying path 11, and the discharge roller 24 is disposed at an outlet thereof.

Details of the fixing unit

The following describes the sequence of the first to third embodiments of the fixing unit 14 used in the image forming apparatus 1.

First embodiment

Fig. 2 is a vertical cross-sectional view showing the structure of the fixing unit 14 according to the first embodiment. Fig. 2 shows a state in which the fixing unit 14 mounted on the image forming apparatus 1 is rotated by about 90 ° counterclockwise. Therefore, the sheet feeding direction from bottom to top in fig. 1 becomes from right to left in fig. 2. In addition, when the apparatus main body 2 is larger (such as a complex machine), it may be mounted in the orientation shown in fig. 2.

The fixing unit 14 has the pressure roller 44, the fixing roller 45, the heating roller 46, and the heating belt 48 as described above. As described above, since the pressure roller 44 is a metal roller and the surface layer of the fixing roller 45 has an elastic layer of silicon sponge, a flat nip NP is formed between the pressure roller 44 and the fixing roller 45. Further, a halogen heater 44a is provided inside the pressure roller 44. The base material of the heating belt 48 is a ferromagnetic material (e.g., Ni), and a thin-film elastic layer (e.g., silicone rubber) is formed on the surface layer thereof, and a release layer (e.g., PFA) is formed on the outer surface thereof. The shaft core of the heating roller 46 is a magnetic metal (e.g., Fe), and a releasing layer (e.g., PFA) is formed on the surface thereof.

The fixing unit 14 conveys the sheet by sandwiching the sheet in a nip NP portion between the pressure roller 44 and the fixing roller 45 via the heating belt 48. In this conveyance process, the pressure roller 44 and the heating belt 48 supply heat to the paper, and the toner image transferred on the paper is fixed on the paper.

In addition, the fixing unit 14 is equipped with an IH coil unit 50 (not shown in fig. 1) outside the heating roller 46 and the heating belt 48. The IH coil unit 50 includes an induction heating coil 52, a pair of arch cores 54, a pair of side cores 56, and a center core 58.

Coil

As shown in fig. 2, the induction heating coil 52 is disposed on a virtual arc surface along the arc outer surface in order to inductively heat the arc portions of the heating roller 46 and the heating belt 48. The induction heating coil 52 extends in the longitudinal direction of the heat roller 46, and covers substantially the entire longitudinal direction of the heat roller 46. Actually, a resin cover, not shown, for example, is disposed outside the heating roller 46 and the heating belt 48, and the induction heating coil 52 is disposed on the resin cover in a wound state.

Magnetic core

In fig. 2, the center core 58 is located at the center, and the arch core 54 and the side cores 56 are arranged in pairs on both sides thereof. The arch cores 54 on both sides are ferrite cores having symmetrical cross sections and arch shapes, and have a length longer than the winding area 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 area of the induction heating coil 52. The arch core 54 and the side core 56 are fixedly disposed at a plurality of locations in the longitudinal direction of the heat roller 46 with a gap therebetween, for example. The arrangement of the arch cores 54 and the side cores 56 is determined in accordance with, for example, the magnetic flux density (magnetic field intensity) distribution of the induction heating coil 52.

Center magnetic core

The center core 58 is a ferrite core having a cylindrical cross section. The center core 58 is substantially the same as the heat roller 46, and has a length substantially corresponding to the maximum paper passage width (the width of the maximum size paper among the paper conveyed by the fixing unit 14). Although not shown in fig. 2, the center core 58 is connected to a rotating mechanism (see fig. 8A and 8B) and can be rotated about its longitudinal axis by the rotating mechanism. The rotation mechanism will be described later. The cross-section of the center core 58 may also be cylindrical, for example.

The center core 58 is disposed between the arch core 54 and the heating roller 46 (heating belt 48) in the direction of the magnetic field generated by the induction heating coil 52, and forms a magnetic path together with the arch core 54 and the side cores 56. Specifically, the end portion 54a (the inlet portion or the outlet portion of the magnetic circuit) of the arch core 54 is located away from the heating belt 48, and the center core 58 is used to form an intermediate magnetic circuit between the end portion 54a and the heating belt 48.

Temperature control unit

In the example of fig. 2, the temperature control section includes a thermistor 62 (temperature reaction element) and a temperature control circuit 621. The thermistor 62 is provided inside the heat roller 46 in order to detect the temperature of the heat roller 46. One or more thermistors 62 may be disposed inside a portion of the heat roller 46 where the amount of heat generated by induction heating is large. In the first embodiment, the thermistor 62 is preferably disposed inside the axial center position of the heat roller 46 (in the region of the minimum paper passage width W1 shown in fig. 6A and 6B described later).

A temperature control circuit 621 provided in the image forming apparatus 1 controls a power supply unit 521 that supplies an alternating current to the induction heating coil 52, based on the temperature detected by the thermistor 62. The temperature control circuit 621 controls the alternating current supplied from the power supply device 521 to the induction heating coil 52 so that the temperature T detected by the thermistor 62 is maintained at a target temperature Ta required to fix the toner image on the paper. This control may be on-off control of the power supply unit 521, or may be control for increasing or decreasing the amount of ac power supplied to the induction heating coil 52 by changing the voltage or frequency of the ac power generated by the power supply unit 521.

Further, one or more temperature controllers (temperature reaction elements) not shown in the drawings may be provided inside the heating roller 46. The temperature controller may be disposed inside the heating roller 46, particularly, a portion where the amount of heat generated by induction heating is large, and may operate in response to an excessive temperature rise of the heating roller 46 to stop heating by the induction heating coil 52.

Shielding component

A shield member 60 is mounted on the center core 58 along its outer surface. The shield member 60 is formed in a thin plate shape and is bent into a circular arc shape as a whole in accordance with the outer surface shape of the center core 58. As shown in the figure, the shielding member 60 may be embedded in the wall thickness portion of the center core 58, or may be adhered to the outer surface of the center core 58. For attaching the shield member 60, for example, a silicon adhesive can be used. In the fixing unit 14 of the first embodiment, the shielding member 60 constitutes a magnetic shielding portion together with the center magnetic core 58.

The material constituting the shield member 60 is preferably nonmagnetic and excellent in conductivity, and may be, for example, oxygen-free copper. The shield member 60 generates a counter magnetic field by an induced current generated by a vertical magnetic field penetrating the surface thereof, and shields the magnetic field by canceling the interlinkage flux (vertical penetrating magnetic field). Further, by using a material having excellent conductivity, joule heat generated by the induced current can be suppressed, and the magnetic field can be effectively shielded. For example, effective methods for improving conductivity include: (1) selecting a material having a low intrinsic resistance as much as possible, and (2) increasing the thickness of the member. Specifically, the plate thickness of the shield member 60 is preferably in the range of 0.5mm to 3mm, and in the present embodiment, for example, 1 mm. This makes it possible to sufficiently reduce the intrinsic resistance of the shield member 60 and obtain a sufficient magnetic shielding effect, while reducing the weight of the shield member 60.

Magnetic adjustment member

In the IH coil unit 50, magnetic adjustment members 90 are provided between the induction heating coil 52 and the heating roller 46 on both sides of the center core 58. In the cross section of fig. 2, each magnetism adjustment member 90 is provided at a position overlapping with the winding area of the induction heating coil 52. These magnetic adjustment members 90 are all of a nonmagnetic metal (e.g., oxygen-free copper) having a rectangular ring shape in plan view. Since the magnetic adjustment member 90 has a ring shape, the inside thereof is hollowed (hollow), and the sectional shapes of both end portions are shown in fig. 2. However, as shown in fig. 2, the magnetic adjustment member 90 is bent into a circular arc shape as a whole. The center of curvature of the magnetic adjustment member 90 is substantially coincident with the center of rotation of the heat roller 46, for example, and the radius of curvature is smaller than the imaginary arc surface on which the induction heating coil 52 is disposed.

Fig. 3 is a perspective view showing a structural example (1) of the magnetism adjustment member 90. As described above, the magnetic adjustment member 90 is formed as a rectangular ring as a whole, and four sides thereof are constituted by the pair of straight portions 90a opposed in the width direction and the pair of arc portions 90b opposed in the longitudinal direction. In fig. 3, only one magnetic adjustment member 90 is shown, but in the IH coil unit 50, for example, two magnetic adjustment members 90 are provided at each end in the longitudinal direction of the heating belt 48, and four magnetic adjustment members 90 are provided in total. The magnetism adjustment member 90 is attached (fixed) to the inner surface of the resin cover (on which the linear induction heating coil 52 is disposed).

Fig. 4 is a perspective view showing a structural example (2) of the magnetism adjustment member 90. In this configuration example (2), the flange portion 90c is formed on the pair of circular arc portions 90 b. This increases the cross section of the magnetism adjustment member 90 in the circumferential direction, improves the rigidity of the entire structure, and reduces the electric resistance. Further, the pair of linear portions 90a may be formed with flange portions.

Characteristics of magnetic adjustment member

Fig. 5A to 5C are schematic diagrams for explaining the characteristics of the magnetic adjustment member 90. In fig. 5A to 5C, the magnetism adjustment member 90 is simplified to be in the form of a wire.

If a magnetic field (interlinkage magnetic flux) is generated in the ring-shaped magnetic adjustment member 90 so as to penetrate through the ring surface (virtual plane) in the vertical direction (one direction), an induced current is generated in the circumferential direction of the magnetic adjustment member 90 as shown in fig. 5A. Then, a magnetic field (counter magnetic field) opposite to the through magnetic field is generated by the electromagnetic induction, and they cancel each other out, thereby canceling the magnetic field. In the present embodiment, when the shield member 60 is moved to the shielding position, the magnetic shielding effect is compensated by the demagnetizing field effect.

As shown in the upper layer of fig. 5B, it is assumed that a bidirectional through magnetic field is generated on the ring surface of the ring-shaped magnetic adjustment member 90, and the total sum of the interlinkage magnetic fluxes is approximately balanced to 0(± 0). In this case, an induced current is hardly generated in the magnetic adjustment member 90. Therefore, the magnetic adjustment member 90 hardly exerts the effect of canceling the magnetic field, and the magnetic field in both directions passes through the magnetic adjustment member 90 as it is. This is the same as the case where the magnetic field passes through the inside of the magnetism adjusting member 90 in the U-turn direction shown in the lower layer. In the present embodiment, when the shield member 60 is moved to the retracted position, the magnetism adjustment member 90 is fixedly disposed at a position where the magnetic field balance in the loop passing through the magnetism adjustment member 90 is equalized, thereby suppressing the influence on the magnetic field.

Fig. 5C shows a case where a magnetic field (interlinkage magnetic flux) substantially parallel to the ring surface of the ring-shaped magnetic adjustment member 90 is generated. In this case as well, since almost no induced current is generated in the magnetic adjustment member 90, the effect of canceling the magnetic field is not generated. This form is not adopted in the present embodiment.

The inventors of the present invention paid attention to the fact that the magnetic shielding effect and the effect of not shielding the magnetism can be obtained in the state shown in fig. 5A and 5B, and the magnetism adjusting member 90 is fixedly disposed at the most appropriate position to assist the magnetic shielding effect by the shielding member 60 and to prevent the influence on the magnetic field when the shielding member 60 is retracted. Here, first, a method of magnetically shielding by the shielding member 60 will be described.

Method of magnetic shielding

As shown in fig. 2, if the shielding member 60 is located at a position (shielding position) close to the surface of the heating belt 48, the magnetic resistance increases and the magnetic field strength decreases around the induction heating coil 52. On the other hand, if the center core 58 is rotated by 180 ° from the state shown in fig. 2 (the direction is not particularly limited), and the shielding member 60 is moved to the position (retracted position) farthest from the heating belt 48, the magnetic resistance is lowered around the induction heating coil 52, a magnetic path is formed around the center core 58 by the arch cores 54 and the side cores 56 on both sides thereof, and the magnetic field acts on the heating belt 48 and the heating roller 46.

Fig. 6A and 6B are diagrams showing examples of the arrangement of the shield member 60 and the magnetism adjustment member 90. Fig. 6A shows the retracted position, and fig. 6B shows the shielding position. Further, fig. 6A, 6B show side and bottom views of the center core 58 and the magnetism adjustment member 90, respectively. In the figure, the outer surface of the central core 58 is shaded.

As described above, the length of the center core 58 is substantially the same as or longer than the maximum sheet passing width W3 perpendicular to the sheet conveying direction. In this case, the shield member 60 is divided into two in the longitudinal direction of the center core 58, and the shapes thereof are symmetrical to each other. Each shield member 60 is formed into a triangular shape in plan view or bottom view, for example, and a portion corresponding to the apex of the triangular shape is located near the center of the center core 58. That is, the length of the shielding member 60 is shortest near the center of the center core 58 in the circumferential direction of the center core 58, and gradually increases from there toward both side ends of the center core 58.

The shield members 60 are disposed on both outer sides of the minimum paper passing width W1 perpendicular to the paper passing direction, and only a small portion of the shield members 60 is present within the range of the minimum paper passing width W1. The shield members 60 reach the ends of the center core 58 slightly outside the maximum paper passage width W3. The minimum paper passage width W1 and the maximum paper passage width W3 are determined according to the minimum size or the maximum size of paper that can be printed in the image forming apparatus 1. In addition, the size of the paper means a width perpendicular to the paper conveying direction.

In the present embodiment, the ratio of the length of the shielding member 60 to the outer circumference of the center core 58 in the rotational direction of the center core 58 differs along the axial direction (longitudinal direction) of the center core 58. When the ratio of the length (Lc) of the shielding member 60 to the outer circumference (L) of the center core 58 is defined as a coverage ratio (Lc/L), the coverage ratio is small on the inner side of the center core 58 and increases from the inner side to the outer side (both ends) in the axial direction. Specifically, the coverage is smallest near the minimum paper passing area (the range of the minimum paper passing width W1) and largest at both ends of the center core 58 (the range exceeding the maximum paper passing width W3). The passage area refers to an area in contact with the heating belt 48 when the paper passes through the heating belt 48 while being sandwiched between the pressure roller 44 and the fixing roller 45, and in the passage area, a first area not in contact with the paper and a second area in contact with the paper are set in accordance with the paper passage widths W1 to W3. For example, when a sheet having the smallest sheet passing width W1 is conveyed, the vicinity of the center of the heating belt 48 is set as the second region and the vicinity of the both ends of the heating belt 48 are set as the first region in the longitudinal direction of the heating belt 48. When the paper having the maximum paper passage width W3 is conveyed, substantially the entire heating belt 48 is set as the second region in the longitudinal direction of the heating belt 48. In this case, the first region is not set.

The setting according to the paper size (paper passing width) is achieved by switching the position of the shielding member 60 to partially suppress the generated magnetic flux. At this time, the rotation angle (rotational displacement amount) of the center core 58 is changed depending on the paper size, and the shielding amount is decreased as the paper size is larger, and the shielding amount is increased as the paper size is smaller, whereby the both end portions of the heating roller 46 and the heating belt 48 can be prevented from being excessively heated. In fig. 6A and 6B, the counterclockwise rotation is shown by an arrow, but the center core 58 may be rotated clockwise. Further, the paper conveying direction may also be opposite to the direction shown in fig. 6A, 6B.

Of magnetically-adjustable membersSegmentation

In the example shown in fig. 6A and 6B, the entire magnetism adjustment member 90 is a rectangular ring, and the inside thereof is divided (divided) into a plurality of regions by the circular arc portion 90B. Therefore, the magnetic adjustment member 90 is divided into a plurality of loop portions in the longitudinal direction of the heating belt 48 (i.e., the direction perpendicular to the paper conveying direction). Thus, the structure of dividing one ring into a plurality of regions can accommodate the paper passing widths W1, W2, and W3 which are different depending on the paper size. For example, in the case where the paper size is minimum (the minimum paper passage width W1), the magnetic shielding effect can be compensated with all three ring portions of each magnetic adjustment member 90. In addition, in the case where the paper size is in the range from the minimum to the middle (the minimum paper passage width W1 to the middle paper passage width W2 or less), the magnetic shielding effect can be compensated by the two ring portions from the outer side of each magnetic adjustment member 90. When the paper size is the maximum (the maximum paper passage width W3), all of the three loop portions of each magnetic adjustment member 90 do not generate an induced current, and do not affect the magnetic field generated by the induction heating coil 52.

Other ways of magnetically adjusting the members

Fig. 7 is a diagram showing an example of the arrangement of the magnetism adjustment member 90 according to another embodiment. The magnetic adjustment members 90 shown in fig. 7 are independently arranged in a direction perpendicular to the sheet conveying direction. That is, the magnetic adjustment members 90 are rings and are not electrically connected to each other. In this case, the paper passing widths W1, W2, and W3, which are different depending on the paper size, can be also accommodated. For example, in the case where the paper size is minimum (minimum paper passing width W1), the magnetic shielding effect can be compensated with the entire magnetic adjustment member 90. In addition, in the case where the paper size ranges from the minimum to the middle (the minimum paper passage width W1 to the middle paper passage width W2 or less), the magnetic shielding effect is compensated by two (eight in total) magnetic adjusting members 90 from both outer sides, respectively. When the paper size is the maximum (the maximum paper passage width W3), no induction current is generated in all the magnetism adjustment members 90, and the magnetic field generated by the induction heating coil 52 is not affected.

Rotation mechanism of magnetic shield part

A mechanism for rotating the center core 58 about the axis, that is, a mechanism for changing the position of the shielding member 60 between the shielding position and the retracted position will be described with reference to fig. 8A and 8B. Fig. 8A is a side view showing the structure of the rotating mechanism 64 of the center core 58, and fig. 8B is a cross-sectional view taken along the direction B-B in fig. 8A.

As shown in fig. 8A and 8B, the rotation mechanism 64 includes a stepping motor 66, a reduction mechanism 68, and a drive shaft 70. The rotation mechanism 64 reduces the rotation of the stepping motor 66 to a predetermined rotation speed by the reduction mechanism 68, drives the drive shaft 70, and rotates the center core 58 about the axis thereof. The axis of the center core 58 extends in a direction intersecting the direction in which the magnetic field generated by the induction heating coil 52 passes through the center core 58. The speed reduction mechanism 68 is, for example, a worm, but other means may be used. In order to detect the rotation angle (the amount of rotational displacement from the reference position) of the center core 58, a notched disk 72 is provided at the end of the drive shaft 70, on which a photo interrupter (photo interrupter)74 is combined.

The drive shaft 70 is connected to one end of the center core 58, does not penetrate the inside of the center core 58, and supports the center core 58. The angle of rotation of the center core 58 can be controlled by the number of drive pulses applied to the stepping motor 66, for which purpose a control circuit (not shown) is provided in the rotating mechanism 64. The control circuit may be constituted by, for example, a control IC, an input/output driver, a semiconductor memory, and the like. The detection signal from the photointerrupter 74 is input to the control IC via the input driver, and the control IC detects the current rotation angle (position) of the center core 58 based on the input signal. On the other hand, information on the current paper size from an image formation control unit not shown in the figure is notified to the control IC. The control IC receives the information, reads information of a rotation angle suitable for a paper size from a semiconductor memory (ROM), and outputs a drive pulse corresponding to the arrival at the target rotation angle at a predetermined cycle. The drive pulse is applied to the stepping motor 66 through the output driver, and the stepping motor 66 is operated.

Fig. 9A and 9B are diagrams showing an example of the operation accompanying the rotation of the center core 58. The following description will be made separately.

Allowed state

Fig. 9A shows an operation example when the shield member 60 is switched to the retracted position with the rotation of the center core 58. In this case, the magnetic field generated by the induction heating coil 52 passes through the heating belt 48 and the heating roller 46 via the side cores 56, the arch cores 54, and the center core 58. At this time, eddy currents are generated in the heating belt 48 and the heating roller 46, which are ferromagnetic bodies, and joule heat is generated by the intrinsic resistances of the respective materials, thereby heating the heating belt 48 and the heating roller 46.

Function of magnetic adjusting member (1)

At this time, the magnetic path passes through the heating belt 48 and the heat roller 46 via the side cores 56, the arch cores 54, and the center core 58, and on the inside of such a magnetic path, for example, the magnetic flux leaking from the arch cores 54 passes through the inside of the loop of the magnetism adjusting member 90, passes through the heating belt 48 and the heat roller 46, and then passes through the inside of the loop of the magnetism adjusting member 90 to be collected in the arch cores 54. The magnetic flux thus leaked does not pass through the center core 58 or the side cores 56, but contributes to induction heating of the heating belt 48 and the heating roller 46 inside the magnetic path.

The magnetic adjustment member 90 is in a state shown in the lower layer of fig. 5B with respect to the leaked magnetic flux. That is, since the magnetic field passes through the ring inside of the magnetism adjusting member 90 in the U-turn direction, the magnetism adjusting member 90 does not exert a canceling effect on the leaked magnetic flux, allowing it to pass (second state). Therefore, induction heating of the heating belt 48 is not hindered, and the preheating time can be shortened.

Shielded state

Fig. 9B shows an operation example when the shielding member 60 is switched to the shielding position. In this case, since the shielding member 60 is located on the magnetic path outside the minimum paper passing area, the generation of the magnetic field is partially suppressed. Therefore, the amount of heat generation outside the minimum paper passage area is suppressed, and excessive temperature rise of the heating belt 48 and the heating roller 46 can be prevented. Further, by changing the rotation angle of the center core 58 little by little, the shielding amount of the magnetic field can be adjusted. For example, if the rotation angle of the center core 58 is increased counterclockwise from the position shown in fig. 9B, the magnetic field is generated without being shielded on the left side in the figure, but the magnetic field is continuously shielded on the right side in the figure. In this case, the magnetic field intensity generated as a whole is lower than that at the position shown in fig. 9A, and therefore the amount of heat generation at this portion can be reduced.

Function of magnetic adjusting member (2)

At this time, the magnetic paths passing through the heating belt 48 and the heat roller 46 via the side cores 56, the arch cores 54, and the center core 58 are shielded, and on the inner side thereof, for example, the magnetic flux leaking from the arch cores 54 passes through the heating belt 48 and the heat roller 46 via the inner side of the loop of the magnetism adjusting member 90. If the leaked magnetic flux inductively heats the heating belt 48 and the heating roller 46, the shielding effect of the shielding member 60 is reduced.

In the present embodiment, the magnetism adjustment member 90 is in the state shown in fig. 5A with respect to the magnetic flux leaking from the arch core 54 in the shielded state. That is, since the leaked magnetic flux is interlinkage magnetic flux inside the ring of the magnetic adjustment member 90, the magnetic adjustment member 90 can exert an effect of canceling the leaked magnetic flux and can suppress the passage thereof (first state). The magnetic adjusting member 90 can compensate the shielding effect of the shielding member 60, so that a sufficient magnetic shielding effect can be obtained without increasing the area of the shielding member 60 very much, and excessive temperature rise of the heating belt 48 and the heating roller 46 can be further suppressed as compared with the conventional case.

As described above, the magnetic shielding portion constituted by the shielding member 60 and the center core 58 functions as a switching portion for switching between the first state in which the magnetic adjusting member 90 exhibits the magnetic shielding effect when the shielding member 60 is positioned at the shielding position and the second state in which the magnetic adjusting member 90 does not exhibit the magnetic shielding effect when the shielding member 60 is positioned at the retracted position.

Condition setting

The following examples illustrate setting conditions found by the inventors of the present invention. First, it is preferable that the magnetism adjusting member 90 is a member having non-magnetic properties and excellent conductivity, so that it is possible to suppress joule heat generated by the induced current and effectively shield magnetism when the magnetic shielding effect of the shielding member 60 is compensated as described above. In this embodiment, as described above, a material such as oxygen-free copper is used. In addition, in order to improve the conductivity of the magnetism adjustment member 90, a material having a small intrinsic resistance is selected as much as possible, and the thickness of the material (symbol t in the drawing) needs to be increased. Under the conditions found by the inventors of the present invention, the thickness t of the magnetism adjustment member 90 is preferably set in the range of 0.5mm to 3 mm.

Fig. 10 is a diagram showing various conditions set in the present embodiment. The inventors of the present invention present the following conditions regarding the angle (symbol α in the figure) at which the magnetic adjusting member 90 is provided with respect to the heating belt 48 and the heat roller 46. For example, the center is the axis of the heat roller 46, and a horizontal line (a horizontal line in fig. 10, different from an actual mounting state) passing through the center is a reference angle (═ 0 °). If the angle (deg) of the ring center L of the magnetic adjustment member 90 is acquired from the center in the counterclockwise direction, the optimum condition can be set in the following logic with respect to the angle α at which the magnetic adjustment member 90 is set and its ring width WR.

Fig. 11 is a distribution graph showing the intensity distribution of the radial magnetic field around the heat roller 46 (360 °). In fig. 11, the horizontal axis represents an angle (deg) of rotation counterclockwise from the reference angle (0 °), and the vertical axis represents, for example, a radial magnetic field (a/m). The curve indicated by the thick solid line in fig. 11 shows the distribution when the shielding member 60 is not magnetically shielded (the retracted position). In fig. 11, a curve indicated by a broken line shows a distribution when the shielding member 60 performs magnetic shielding (shielding position). In this case, the arrangement of the magnetic adjustment member 90 is preferably such that the magnetic field is not affected without performing magnetic shielding.

However, if it is assumed that the shield member 60 moves to the retracted position and does not exert the effect of canceling the magnetic field (distribution of thick solid lines in the figure), the radial magnetic field has peaks in intensity near 0 °, near 90 °, and near 180 °, and almost no magnetic field is generated near 270 ° (directly below) where the induction heating coil 52 is not disposed.

In such a magnetic field distribution, first, a point P where the radial magnetic field is 0 is taken on the distribution curve, and angles α 1 and α 2 having the same value (the area S1 in the figure is S2) are obtained by integrating the distribution curve in the 0 ° direction and the 90 ° direction from the point P. It can be seen that if the magnetic adjustment member 90 is disposed within the range of the obtained angles α 1 to α 2, the magnetic field balance of the magnetic adjustment member 90 becomes 0 without performing magnetic shielding, and as a result, the magnetic field is not disturbed.

Then, an angle (═ α 1+ α 2)/2) at the midpoint between the angles α 1 and α 2 on the horizontal axis is determined, and this angle is taken as the angle of the ring center L at the retreat position. If the distance from the center of the heat roller 46 to the magnetic adjustment member 90 is defined as the radius r, the virtual arc length with the radius r and within the range of the angles α 1 to α 2 is the optimum loop width of the magnetic adjustment member 90. In this example, the ring width WR can be obtained by the following equation. The peripheral radius r is larger than the radius (D/2 in the drawing) of the heat roller 46.

WR＝r×{(α2—α1)/180}×π

As described above, the optimum condition can be set by setting the loop width of the magnetism adjustment member 90 to WR (expression above) and setting the angle of the loop center to (α 1+ α 2)/2. In particular, in the state where the shield member 60 is switched to the retracted position, the magnetic field is exactly 0 in the loop inner side (area S1 is S2), and therefore the magnetic field generated by the induction heating coil 52 is not hindered. In the first embodiment, the ring width WR is preferably selected from the range of 1mm to 5 mm. Selecting the loop width WR within such a range helps to make the inherent resistance of the magnetic adjustment member 90 sufficiently small. The loop width WR is a distance between the pair of linear portions 90a, 90a of the magnetic adjustment member 90.

First to fourth modified examples of the fixing unit 14 according to the first embodiment will be described below. Note that the same reference numerals are used for the same components as those in the first embodiment, and redundant description is omitted. Note that even if the reference numerals are the same, if the materials and the like are different, supplementary description thereof will be given.

First modification

Fig. 12 is a diagram showing a first modification of the fixing unit 14 according to the first embodiment. In this first modification, the toner image is fixed by the fixing roller 45 and the pressure roller 44 without using the heating belt 48 and the heat roller 46. A magnetic material similar to the heating belt is wound around the outer circumference of the fixing roller 45, and the magnetic material is inductively heated by the induction heating coil 52. In this case, the thermistor 62 is disposed at a position opposite to the magnetic layer on the outer side of the fixing roller 45. Otherwise, the amount of shielding of the magnetic field can be adjusted by rotating the center core 58, as described above. Further, the magnetism adjusting member 90 is disposed between the induction heating coil 52 and the fixing roller 45.

Second modification example

Fig. 13 is a longitudinal sectional view showing a second modification of the fixing unit 14 according to the first embodiment. In the second modification, the heating roller 46 is made of a non-magnetic metal (e.g., SUS: stainless steel) material, and is different from the first modification in that the center core 58 is disposed inside the heating roller 46. Further, an arch core 54 is connected to the center, and an intermediate core 55 is provided 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 penetrates the heat roller 46 to the inner center core 58 via the side cores 56, the arch cores 54, and the center core 55. The heating belt 48 is inductively heated by a through magnetic field.

In the second modification, as shown in fig. 13, the state in which the shield member 60 is away from the intermediate core 55 is the retracted position, and in this case, the magnetic shield effect cannot be exerted, and the heating belt 48 is inductively heated in the maximum paper passage area. On the other hand, when the shielding member 60 is switched to a position (shielding position) opposing the intermediate core 55, magnetic shielding is performed to suppress excessive temperature rise outside the paper passing region.

Third modification example

Fig. 14 is a longitudinal sectional view showing a third modification of the fixing unit 14 of the first embodiment. In the third modification, the heat roller 46 is made of a magnetic shunt metal (e.g., an iron-nickel alloy), and is different from the first and second modifications in that the heat roller 46 itself is magnetically shielded by a temperature change. That is, in the third modification, after the heat roller 46 is heated to the curie temperature or higher, magnetism is lost and the magnetic shielding effect is exhibited, so that it is possible to prevent the heat roller 46 itself from being excessively heated. On the other hand, while the heat roller 46 is heated in the range of the curie temperature or lower, the toner image can be fixed by the joule heat of the heat roller 46 because the magnetic conductivity is increased by the magnetism of the permanent magnet metal. Further, the magnetism adjustment member 90 is disposed between the induction heating coil 52 and the heating belt 48.

In the case of the third modification, since the shield member is not necessary, the center core for providing the shield member is not necessary, and the rotation mechanism of the shield member is not necessary, so that the structure can be simplified.

Fourth modification example

Fig. 15 is a diagram showing a fourth modification of the fixing unit 14 according to the first embodiment. In the fourth modification, induction heating is performed not at the circular arc-shaped position of the heating belt 48 but at the flat position between the heating roller 46 and the fixing roller 45. In this case, the amount of shielding of the magnetic field can also be adjusted by rotating the center core 58. Further, the magnetism adjustment member 90 is a planar ring, and is disposed between the induction heating coil 52 and the heating belt 48.

The first embodiment and the first to fourth modifications described above can be modified in various ways. For example, the cross-sectional shape of the center core 58 is not limited to a cylinder or a cylinder, and may be a polygon. The shape of the shielding member 60 in plan view is not limited to a triangle, and may be a trapezoid.

The shape, size, number of divisions, and the like of the ring of the magnetic adjustment member 90 are not limited to the examples.

In addition, the specific form of each portion including the arch core 54 and the side cores 56 is not limited to the form in the drawing, and may be changed as appropriate.

Second embodiment

The fixing unit 14 according to the second embodiment will be described in detail below. Fig. 16 is a longitudinal sectional view showing the fixing unit 14 of the second embodiment. Fig. 2 shows a state in which the fixing unit 14 actually mounted on the image forming apparatus 1 is rotated by about 90 ° counterclockwise. Therefore, the paper conveying direction from bottom to top in fig. 1 is from right to left in fig. 16. Further, when the apparatus main body 2 is larger (such as a complex machine), it may be mounted in the orientation shown in fig. 16.

The fixing unit 14 includes a pressure roller 44, a fixing roller 45, and a heating belt 148 wound around its outer circumference. Since the pressure roller 44 is a metal roller and the surface layer of the fixing roller 45 (the inner side of the heating belt 148) has an elastic layer of silicone sponge, a flat nip NP is formed between the pressure roller 44 and the fixing roller 45. Further, a halogen heater 44a is provided inside the pressure roller 44. The base material of the heating belt 148 is a ferromagnetic material (e.g., Ni), and a thin film elastic layer (e.g., silicone rubber) is formed on the surface layer thereof, and a release layer (e.g., PFA) is formed on the outer surface thereof.

The fixing unit 14 further includes an IH coil unit 50 disposed outside the fixing roller 45 (heating belt 148). The IH coil unit 50 includes an induction heating coil 52, an arch core 54, and a pair of side cores 56. The arch core 54 and the side cores 56 are magnetic bodies obtained by sintering ferrite powder, for example. In which the arch core 54 is divided into three parts, but the arch core 54 may be formed as a single body.

Coil

As shown in fig. 16, the induction heating coil 52 is disposed on a virtual arc surface along the arc outer surface in order to perform induction heating at the arc portion of the fixing roller 45 (heating belt 148). Actually, a resin cover (not shown) is disposed outside the fixing roller 45 (heating belt 148), for example, and the induction heating coil 52 is disposed on the resin cover by winding.

Winding center

Although not shown in the drawings, the induction heating coil 52 is wound into an oval shape in a plan view (a state viewed from above in fig. 16). That is, the length of the fixing roller 45 (heating belt 148) covers the maximum paper passage width, and the winding area of the induction heating coil 52 is also slightly longer than the entire length of the fixing roller 45 and the like in order to generate a magnetic field over substantially the entire area in the longitudinal direction thereof. Therefore, the induction heating coil 52 can inductively heat substantially the entire region in the longitudinal direction of the fixing roller 45 and the like. On the other hand, in the cross section shown in fig. 16, a magnetic field may be generated at substantially the half of the fixing roller 45 or the like. The induction heating coil 52 can thus inductively heat substantially a half circumferential portion in the circumferential direction of the fixing roller 45 or the like. The induction heating coil 52 is formed around the winding center (indicated by symbol C in the drawing) as viewed in the cross section of fig. 16, and the winding center substantially coincides with the center line of the fixing roller 45 and the like. In the description of the second embodiment and the third embodiment described later, the "winding center" is referred to as a center line (symbol C) on the cross section shown in fig. 16.

In the second embodiment, a thermistor 62 (or a temperature controller) is provided outside the fixing roller 45 so as to face the fixing roller 45. The thermistor 62 may be disposed, for example, at a position where the magnetic field intensity generated by the induction heating coil 52 is high and where it does not interfere with the induction heating coil 52.

Magnetic adjustment member

A magnetism adjustment member 160 is provided between the induction heating coil 52 and the arch core 54 in the IH coil unit 50. The magnetic adjustment member 160 is a non-magnetic metal (e.g., oxygen-free copper) and has a rectangular ring shape in plan view. Since the magnetic adjustment member 160 has a ring shape, the inside thereof is hollowed (hollow), and only the cross-sectional shapes of both ends thereof are shown in fig. 16. The magnetic adjustment member 160 is bent in a circular arc shape as a whole. The center of curvature of the magnetism adjustment member 160 substantially coincides with the center of rotation of the fixing roller 45, for example, has a radius of curvature larger than that of an imaginary arc surface on which the induction heating coil 52 is disposed, and does not interfere with the induction heating coil 52 in length.

Fig. 17 is a perspective view showing a structural example (1) of the magnetism adjustment member 160. As described above, the magnetic adjustment member 160 is formed as a rectangular ring as a whole, and its four sides are formed by the pair of straight portions 160a opposed in the width direction and the pair of arc portions 160b opposed in the longitudinal direction.

Magnetic adjustment part

The magnetic adjustment unit includes a support member 164 that supports the magnetic adjustment member 160, a drive shaft 166 attached to the support member 164, and a drive mechanism (a stepping motor and a reduction mechanism) not shown in the drawing connected to the drive shaft 166. The magnetic adjustment member 160 is supported at one end in the longitudinal direction thereof by a support member 164, for example. The support member 164 has, for example, a fan-shaped side plate 164a and a circular arc-shaped top plate 164b, wherein the top plate 164b is closely attached to the lower surface of one circular arc portion 160b of the magnetic adjustment member 160. In fig. 17, a side plate 164a extends downward from the top plate 164b, and a drive shaft 166 is mounted on a major portion thereof. The magnetic adjustment part, if the driving shaft 166 is rotated by the driving mechanism, can change the position of the magnetic adjustment member 160 in the rotational direction together with the support member 164. This moves the magnetism adjustment member 160 to a shield position or a retracted position described later.

Fig. 18 is a perspective view showing a structural example (2) of the magnetism adjustment member 160. In this configuration example (2), the flange portion 160c is formed on the pair of circular arc portions 160 b. This makes it possible to increase the cross section of the magnetism adjustment member 160 in the circumferential direction, thereby improving the overall rigidity. Further, the pair of linear portions 160a may be formed with flange portions. Note that, although not shown in the drawings, the magnetic adjustment member 160 may be supported by the same support member as in structural example (1), and the position of the magnetic adjustment member 160 may be changed by a drive mechanism.

Principle of magnetic shielding effect

The principle of achieving the magnetic shielding effect by the magnetic adjusting member 160 is the same as the principle of achieving the magnetic shielding effect by the magnetic adjusting member 90 described with reference to fig. 5A to 5C, and therefore, a detailed description thereof is omitted. In the second embodiment, the optimal magnetic adjustment is performed by moving the magnetic adjustment member 160 to the shielding position or the retracted position. An example of the magnetic adjustment method will be described below.

Examples of the method of adjusting the magnetic field

Fig. 19A and 19B are diagrams illustrating a method of performing magnetic adjustment by the magnetic adjustment member 160. Fig. 19A shows a state (first state) in which the magnetic adjustment member 160 is located at the shielding position, and fig. 19B shows a state (second state) in which the magnetic adjustment member 160 is located at the retracted position.

Shielding position

As shown in fig. 19A, generally, when the induction heating coil 52 is energized, a magnetic path is formed around the induction heating coil through the side cores 56 and the arch cores 54, and longitudinally passes through an air gap at the position of the winding center C to reach the heating zone 148. In this case, if the magnetism adjusting member 160 is provided in the middle of the magnetic path passing through the winding center C by the magnetism adjusting portion, the magnetic field can be canceled according to the principle shown in fig. 5A, so that the magnetism can be shielded within the range of the magnetism adjusting member 160. In the cross section shown in the drawing, at such a shielding position, the ring center (center line of the torus) of the magnetic adjustment member 160 and the winding center (symbol C) substantially coincide on one axis (first state).

Retreat position

As shown in fig. 19B, if the position of the magnetism adjustment member 160 is shifted from the winding center C by the magnetism adjustment unit, for example, to a position overlapping with the winding region of the one-side induction heating coil 52, the shielding of the magnetic path passing through the winding center C is released, and a magnetic field can be generated satisfactorily. In this case, the magnetic adjustment member 160 hardly exerts an effect of canceling the magnetic field according to the principle shown in the lower layer of fig. 5B. Therefore, the magnetic field strength of the induction heating coil 52 is not hindered in the state where the position of the magnetism adjustment member 160 is shifted to the retracted position. This allows the heating belt 148 to be efficiently induction-heated, and shortens the preheating time. In such a retracted position, the ring center (symbol L in the drawing) of the magnetic adjustment member 160 and the winding center C do not coincide on one axis, that is, are in a non-coaxial relationship (second state).

In this way, the magnetic adjuster can function as a switching unit that switches between a first state, which is a shielding position where the magnetic adjuster 160 exerts the magnetic shielding effect, and a second state, which is a retracted position where the magnetic adjuster 160 cannot exert the magnetic shielding effect.

Condition setting

Next, the condition setting of the fixing unit 14 according to the second embodiment will be described. First, in order to effectively shield magnetic flux by suppressing joule heat generated by induced current, it is preferable that the magnetic adjusting member 160 is a member having non-magnetic properties and good electrical conductivity, and as described above, a material such as oxygen-free copper can be used. In addition, in order to improve the conductivity of the magnetism adjustment member 160, it is necessary to select a material having a small intrinsic resistance as much as possible and to increase the thickness of the material. According to the conditions found by the inventors of the present invention, the thickness of the magnetism adjustment member 160 is preferably selected from the range of 0.5mm to 3mm, and in this example, the thickness is 1 mm.

The inventors of the present invention present the following conditions with respect to the angle of changing the position of the magnetism adjustment member 160. As shown in fig. 19B, the axis of the fixing roller 45 is set as a center, and a horizontal line (a horizontal line in fig. 6, which is different from an actually mounted state) passing through the center is set as a reference angle (═ 0 °). If the angle (deg) of the ring center L of the magnetic adjustment member 160 is taken from the center in the counterclockwise direction, the optimum shield position is 90 °. On the other hand, the optimum condition for the angle α of the retracted position can be set according to the following logic.

Fig. 20 is a distribution graph showing the intensity distribution of the radial magnetic field around the fixing roller 45 (360 °). As shown in fig. 20, the horizontal axis represents an angle (deg) of rotation counterclockwise from the reference angle (0 °), and the vertical axis represents, for example, a radial magnetic field (a/m). However, if it is assumed that the effect of eliminating the magnetic field is not obtained when the magnetism adjustment member 160 is located at the retracted position, the radial magnetic field has peak values of intensity in the vicinity of 0 °, 90 °, and 180 °, respectively, and almost no magnetic field is generated in the vicinity of 270 ° (directly below) where the induction heating coil 52 is not arranged.

In such a magnetic field distribution, first, a point P where the radial magnetic field is 0 is taken on the distribution curve, and angles α 1 and α 2 having the same value (the area S1 in the figure is S2) are obtained by integrating the distribution curve in the 0 ° direction and the 90 ° direction from the point P. It can be seen that if the magnetism adjustment member 160 is disposed within the range of the obtained angles α 1 to α 2, the balance of the magnetic field is 0 at the retracted position, and as a result, the magnetic field is not disturbed.

Then, the angle (═ α 1+ α 2)/2) at the midpoint between the angles α 1 and α 2 on the horizontal axis is determined, and this is taken as the angle of the ring center L at the retreat position. When the distance from the center of the fixing roller 45 to the magnetic adjustment member 160 is defined as the radius r, the virtual arc length within the range of the angle α 1 to α 2 at the radius r is the optimum loop width of the magnetic adjustment member 160. In this example, the loop width WR can be obtained by the following equation.

WR＝r×{(α2—α1)/180}×π

Therefore, the optimal condition can be set by setting the ring width of the magnetism adjustment member 160 to WR (equation above), setting the angle of the ring center at the retreat position to (α 1+ α 2)/2, and setting the ring center angle at the shielding position to 90 °. In particular, since the convergence of the magnetic field inside the loop (area S1 — S2) is exactly 0 at the retracted position, the magnetic field generated by the induction heating coil 52 is not disturbed. In the second embodiment, the ring width WR is preferably selected from the range of 1mm to 5 mm. Selecting the loop width WR within such a range helps to make the inherent resistance of the magnetic adjustment member 90 sufficiently small.

Structural example of magnetic adjustment member

Fig. 21A to 21C are plan views showing various arrangement examples of the magnetic adjustment member. The vertical direction in fig. 21A to 21C corresponds to the longitudinal direction of the fixing roller 45 (i.e., the direction perpendicular to the paper conveying direction).

In the arrangement example shown in fig. 21A, the magnetism adjustment member 160 is a single rectangular ring as described above, and is divided into two in the longitudinal direction. The magnetic adjustment members 160 are provided on both outer sides of the minimum paper passage width W1 perpendicular to the paper conveying direction, and the magnetic adjustment members 160 are provided only a little within the range of the minimum paper passage width W1. The magnetic adjustment member 160 is located at the opposite ends of the fixing roller 45 slightly outside the maximum paper passage width W3. Further, the minimum paper passage width W1 and the maximum paper passage width W3 are determined in accordance with the minimum size or the maximum size of paper that can be printed by the image forming apparatus 1.

In the arrangement example shown in fig. 21B, three magnetic adjustment members 170, 172, 174 are arranged on both sides, respectively. Each of the magnetic adjustment members 170, 172, 174 is a rectangular ring, but the ring width thereof is different in the longitudinal direction. That is, the loop width of the magnetic adjustment member 174 closest to the minimum paper passage area is the smallest, and the loop widths of the magnetic adjustment members 172 and 170 located outside the magnetic adjustment member 174 are sequentially increased.

In the arrangement example shown in fig. 21C, two magnetic adjustment members 180 are arranged on both sides, and the number of the magnetic adjustment members 180 is the same as that of the magnetic adjustment members 160 shown in fig. 21A, but the magnetic adjustment members 180 are different in that they have a trapezoidal shape and bridge portions 180a are provided at two inner portions. That is, each of the magnetic adjustment members 180 has the shortest loop width at a position closer to the center in the longitudinal direction, and gradually increases in loop width from this position toward both side end portions. The bridge portions 180a are arranged at regular intervals in the longitudinal direction, and three rings are formed in one magnetism adjustment member 180 by these bridge portions 180 a.

Active size switching

The paper size is adapted by partially shielding the magnetic field by switching the positions of the magnetic adjustment members 160 to 180. In this case, the positions of the magnetic adjusting members 160 to 180 are changed little by little according to the size of the paper, and the larger the size of the paper is, the smaller the magnetic shielding amount is, and conversely, the smaller the size of the paper is, the larger the magnetic shielding amount is, so that the both end portions of the heating belt 148 can be prevented from being excessively heated. In the following description, the term "magnetism adjustment member 160" is used collectively to prevent complication, but this case also includes the magnetism adjustment members 170, 172, 174, 180, and the like shown in fig. 21A to 21C.

Fig. 22A and 22B are longitudinal sectional views showing modifications of the fixing unit 14 according to the second embodiment. Fig. 22A and 22B show a method of performing magnetic adjustment by the magnetic adjustment member 160. Fig. 22A shows a state where the magnetism adjustment member 160 is positioned at the shielding position, and fig. 22B shows a state where the magnetism adjustment member 160 is positioned at the retracted position.

In the case of this modification, unlike the second embodiment, the magnetism adjustment member 160 moves between the induction heating coil 52 and the fixing roller 45 (heating belt 148). The other configurations are the same as those of the second embodiment, and the same reference numerals are given to the same components as those already described, and redundant description is omitted.

Shielding position

When the induction heating coil 52 is energized, a magnetic path is formed around it through the side cores 56 and the arch cores 54, longitudinally through an air gap at the position of the winding center C, and to the heating belt 148. In this case, as shown in fig. 22A, at a position between the induction heating coil 52 and the fixing roller 45, if the magnetism adjusting member 160 is placed midway in a magnetic path passing through the winding center C, the magnetic field can be eliminated according to the principle shown in fig. 5A. The magnetism can be shielded within the range of the magnetism adjustment member 160. Further, it is preferable that the ring center (center line of the ring surface) of the magnetic adjustment member 160 and the winding center (symbol C) substantially coincide on one axis at the shielding position in the cross-sectional view shown in the drawing.

Retreat position

On the other hand, as shown in fig. 22B, when the position of the magnetism adjustment member 160 is shifted from the winding center C and shifted to a position overlapping with the winding region of the induction heating coil 52 on one side between the induction heating coil 52 and the fixing roller 45, for example, the shielding of the magnetic path passing through the winding center C is released, and a magnetic field can be generated satisfactorily. In this case, the magnetism adjustment member 160 hardly exerts an effect of canceling the magnetic field according to the principle shown in the lower layer of fig. 5B, and therefore does not hinder the magnetic field strength of the induction heating coil 52 at the retracted position. In the retracted position, the ring center (symbol L in the drawing) of the magnetic adjustment member 160 and the winding center C do not coincide with each other on one axis (non-coaxial).

The conditions described in the second embodiment may be adopted for other condition settings. In this modification, since the distance (radius r) from the center of the fixing roller 45 to the magnetism adjusting member 160 is small, the loop width is also reduced accordingly, as compared with the second embodiment.

Fig. 23 is a longitudinal sectional view showing another modification of the fixing unit 14 according to the second embodiment. The fixing unit 14 is provided with a heating roller 46 adjacent to the fixing roller 45, and a heating belt 48 is wound around the heating roller 46 and the fixing roller 45. The core of the heating roller 46 is, for example, iron, and a mold-releasing layer (e.g., PFA) is formed on the surface thereof.

In this modification, induction heating is performed at the circular arc-shaped portions of the heating belt 48 and the heating roller 46. That is, the magnetic field generated by the induction heating coil 52 passes through the heating belt 48 and the heating roller 46 via the side core 56, the arch core 54, and the air gap at the winding center position, and induction-heats them.

In this modification as well, similarly to the second embodiment, by moving the magnetism adjustment member 160 to the shielding position, magnetism can be shielded within the range of the magnetism adjustment member 160. In this way, excessive temperature increases at both end portions of the heating belt 48 and the heat roller 46 can be reliably prevented at the time of switching the size. Further, in the case of a large-sized sheet, by moving the magnetism adjusting member 160 to the retreat position, the magnetic shielding effect can be canceled, and the heating belt 48 and the heating roller 46 can be warmed up as soon as possible.

While fig. 23 shows an example in which the magnetism adjustment member 160 is disposed between the induction heating coil 52 and the heating belt 48 (heating roller 46), the magnetism adjustment member 160 may be disposed between the induction heating coil 52 and the arch core 54 and the position of the magnetism adjustment member 160 may be changed between them as in the second embodiment.

Third embodiment

Fig. 24A and 24B are longitudinal sectional views showing a third embodiment of the fixing unit 14. The third embodiment is different from the first and second embodiments in that the fixing unit 14 has an IH coil unit 50 of an internal package type. In addition, the configuration of the magnetism adjustment member 160 may be the same as that of the second embodiment. Further, the magnetism adjustment member 160 can be switched between the shielding position and the retracted position by the magnetism adjustment portion described with reference to fig. 17.

That is, the fixing unit 14 of the third embodiment includes a pressure roller 44 and a heat roller 46, and fixes the toner image while conveying the paper between their nips. Further, since the heat roller 46 is a metal roller, this structure is suitable for fixing a black-and-white image. However, by providing an elastic layer on the surface layer of the heat roller 46, a flat nip NP can be formed between the heat roller 46 and the pressure roller 44. In this case, the fixing of the full-color image is suitable as in the first and second embodiments.

The IH coil unit 50 of the internal type includes an induction heating coil 52 and a ferrite core 59 disposed inside the heating roller 46. That is, the induction heating coil 52 is disposed along the inner circumferential surface of the heat roller 46, and the winding center thereof extends in the horizontal direction from the axial center of the heat roller 46 in fig. 24A and 24B, for example.

The ferrite core 59 is formed by combining two parts into a T-shaped section. One of which extends along the winding center of the induction heating coil 52 and the other of which extends in the radial direction with the winding center sandwiched therebetween.

Shielding position

In the IH coil unit 50 of the internal type, when the induction heating coil 52 is energized, a magnetic path is formed around the induction heating coil, in which a magnetic field is converged by the ferrite core 59 at a position of the winding center from the ferrite core 59 via a part of the heating roller 46. In such an internally-enclosed IH coil unit 50, as shown in fig. 24A, if the magnetism adjustment member 160 is placed in the middle of the magnetic path through the center of the winding, the magnetic field is canceled according to the principle shown in fig. 5A, and therefore the magnetism can be shielded within the range of the magnetism adjustment member 160. In addition, in the cross-sectional view shown in the figure, the ring center (center line of the torus) of the magnetic adjusting member 160 and the winding center are substantially coincident with each other on one axis at the shielding position (first state).

Retreat position

On the other hand, as shown in fig. 24B, when the position of the magnetism adjustment member 160 is shifted from the winding center, for example, to a position overlapping with the winding region of the induction heating coil 52 on one side (upper side in fig. 24B), the shielding of the magnetic path passing through the winding center is released, and a magnetic field can be generated satisfactorily. In this case, according to the principle shown in the lower layer of fig. 5B, the magnetic field strength of the induction heating coil 52 is not hindered because the magnetic adjustment member 160 hardly exerts an effect of canceling the magnetic field. This allows the induction heating of the heating roller 46 to be performed efficiently, and the warm-up time to be shortened. Further, in the third embodiment, the ring center of the magnetic adjustment member 160 does not coincide with the winding center on one axis, that is, is not coaxial with the winding center (second state).

In the third embodiment, as in the second embodiment, the magnetism can be shielded within the range of the magnetism adjustment member 160 by moving the magnetism adjustment member 160 to the shielding position. In this way, excessive temperature increases at both end portions of the heat roller 46 can be reliably prevented at the time of switching the size. Further, in the case of a large-sized sheet, by moving the magnetism adjusting member 160 to the retreat position, the magnetic shielding effect can be canceled, and the heating roller 46 can be warmed up as soon as possible.

In fig. 24A and 24B, an example in which the magnetism adjustment member 160 is disposed between the induction heating coil 52 and the heat roller 46 is shown, but the ferrite core 59 disposed along the center of the winding may be shortened and the magnetism adjustment member 160 may be disposed inside the induction heating coil 52 by utilizing this space.

The fixing unit 14 of the second and third embodiments may be variously modified. The shape and arrangement of the magnetic adjustment member 160 are not limited to those shown in fig. 21A to 21C, and other shapes and arrangements may be adopted.

In the fixing unit 14 according to the third embodiment, the IH coil unit 50 may be arranged at a planar position in addition to the circular arc position of the heating belt 48. In this case, the entire magnetism adjustment member 160 may be formed in a planar shape, and may be slid in a linear direction between the induction heating coil 52 and the arch core 54 or between the induction heating coil 52 and the heating belt 48 to change the position.

Claims (20)

1. An image forming apparatus, characterized by comprising:

an image forming unit for transferring a toner image onto a sheet; and

a fixing unit including a heating member and a pressing member, and configured to convey the sheet while sandwiching the sheet between the heating member and the pressing member, and fix the toner image to the sheet by using at least heat from the heating member during the conveyance; wherein,

the heating member is set to a first area and a second area according to a size of the sheet, the first area is an area where the sheet conveyed by the fixing unit does not contact when passing, and the second area is an area where the sheet conveyed by the fixing unit contacts when passing,

the fixing unit further includes:

a coil that generates magnetic gas for inductively heating the heating member to form a magnetic field;

a magnetic core made of a magnetic material for forming a magnetic path in the vicinity of the coil;

a magnetic adjustment member which is an annular non-magnetic metal and is disposed on the magnetic path; and

and a switching unit configured to be switchable between a first state in which the magnetic adjustment member generates an induced current due to the magnetic field and performs magnetic shielding in the first region, and a second state in which the magnetic adjustment member does not generate the induced current and does not perform magnetic shielding in the first region.

2. The image forming apparatus according to claim 1,

the coil is disposed along an outer surface of the heating member,

the magnetic core is disposed at a position opposite to the heating member with the coil interposed therebetween,

the magnetic adjustment member is disposed between the coil and the heating member,

the switching portion is a magnetic shield portion capable of switching between a shield state of magnetic shielding and an allowance state of allowing passage of magnetism in the first region,

the magnetic adjustment member is switched to the first state when the magnetic shield portion is switched to the shielding state, and the magnetic adjustment member is switched to the second state when the magnetic shield portion is switched to the permission state.

3. The image forming apparatus according to claim 2, wherein the magnetic shielding portion has a shielding member made of a non-magnetic metal, and the shielding member is switched between a shielding position for performing magnetic shielding, which is an inner side of a magnetic path passing through the magnetic core and the heating member around the coil, and a retracted position for allowing passage of magnetism, which is an outer side of the magnetic path.

4. The image forming apparatus according to claim 3,

the heating member is a ferromagnetic body and is provided with a plurality of heating holes,

the magnetic shielding portion has a movable magnetic core in addition to the shielding member, is provided between the magnetic core and the heating member in a direction of a magnetic field generated by the coil, and is provided with the shielding member along an outer surface of the movable magnetic core.

5. The image forming apparatus according to claim 3,

the heating member is a non-magnetic body,

the magnetic shielding part has a movable magnetic core, is provided inside the heating member, forms a magnetic path penetrating the heating member, and is provided with the shielding member along an outer surface of the movable magnetic core.

6. The image forming apparatus according to claim 2, wherein the magnetic shielding portion is the heating member made of a magnetic metallic material, and magnetically shields the heating member in a magnetic field generated by the coil when the heating member is heated to a curie temperature or higher, and inductively heats the heating member by passing magnetism in a state where the curie temperature is not reached.

7. The image forming apparatus according to claim 1, wherein the magnetic adjustment member is a single square ring shape sharing an outer peripheral portion, and an inner portion of the square ring shape is divided into a plurality of portions in a direction perpendicular to a sheet conveying direction.

8. The image forming apparatus according to claim 1, wherein the magnetic adjustment member includes a plurality of square ring members aligned in a direction perpendicular to a sheet conveying direction.

9. The image forming apparatus according to claim 1,

the coil is disposed along an outer surface of the heating member, formed around a predetermined winding center, and generates the magnetic field,

the magnetic core is disposed at a position opposite to the heating member with the coil interposed therebetween, the magnetic path passing through the center of the winding is formed around the coil,

the magnetic adjustment member has a defined ring center,

the switching unit switches a state of the magnetic adjustment member between a first state and a second state, the first state being a state in which the magnetic adjustment member is shielded by substantially overlapping the ring center and the winding center on one axis in a middle of a magnetic path reaching the heating member through the winding center; the second state is a state in which the magnetism adjustment member is moved away from the magnetic circuit so that the ring center and the winding center are located on different axes, thereby allowing the passage of magnetism.

10. The image forming apparatus according to claim 1,

the coil is disposed along an inner surface of a heating region in a circumferential direction of the heating member, is formed around a predetermined winding center, and generates a magnetic field for inductively heating the heating member in the heating region,

the magnetic core is disposed inside the heating member together with the coil, and forms a magnetic path passing through the winding center of the coil,

the magnetic adjustment member has a defined ring center,

the switching unit switches a state of the magnetic adjustment member between a first state and a second state, the first state being a state in which the magnetic field is shielded by substantially overlapping the ring center and the winding center on one axis in a middle of a magnetic path reaching the heating member through the winding center; the second state is a state in which the magnetism adjustment member is moved away from the magnetic circuit so that the ring center and the winding center are located on different axes, thereby allowing the passage of magnetism.

11. The image forming apparatus according to claim 9, wherein the switching portion enables the magnetic adjustment member to switch positions between the coil and the core.

12. The image forming apparatus according to claim 10, wherein the switching portion enables the magnetic adjustment member to switch positions between the heating member and the coil.

13. The image forming apparatus according to claim 9, wherein the switching portion sets a state in which a current flowing in a circumferential direction of the magnetism adjustment member when the coil generates the magnetic field is substantially 0 as the second state.

14. The image forming apparatus according to claim 10, wherein the switching portion sets a state in which a current flowing in a circumferential direction of the magnetism adjustment member when the coil generates the magnetic field is substantially 0 as the second state.

15. The image forming apparatus according to claim 9, wherein said switching portion reciprocates said magnetic adjustment member between said first state and said second state, and is capable of adjusting a magnetic shielding amount in accordance with a displacement amount therebetween.

16. The image forming apparatus according to claim 10, wherein said switching portion reciprocates said magnetic adjustment member between said first state and said second state, and is capable of adjusting a magnetic shielding amount in accordance with a displacement amount therebetween.

17. The image forming apparatus according to claim 9, wherein the magnetic adjustment member is a square ring shape, is divided into a plurality of square rings in a direction perpendicular to a sheet conveying direction, and is different in ring width in the sheet conveying direction, and is set to be minimum in a vicinity of a minimum sheet passing area which is an area where a minimum-sized sheet of the sheets conveyed by the fixing unit contacts when passing.

18. The image forming apparatus according to claim 10, wherein the magnetic adjustment member is a square ring shape, is divided into a plurality of square rings in a direction perpendicular to a sheet conveying direction, and is different in ring width in the sheet conveying direction, and is set to be minimum in a vicinity of a minimum sheet passing area which is an area where a minimum-sized sheet of the sheets conveyed by the fixing unit contacts when passing.

19. The image forming apparatus according to claim 1, wherein the magnetic adjustment member is formed of a non-magnetic metal conductor made of a copper material, and a width dimension is set to 1mm to 5mm and a thickness dimension is set to 0.5mm to 3 mm.

20. The image forming apparatus according to claim 1, wherein the heating member is a metal roller or a metal belt wound around the metal roller.

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