JP2005321642A - Induction heating type fixing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating type fixing apparatus suppressing unevenness in temperature in the axial direction in a passing region of a fixing roller in which a small size sheet is passed while preventing excessive rise in temperature in a non-passing region of the fixing roller in which the small size sheet is not passed. <P>SOLUTION: In the induction heating type fixing apparatus, when the small size sheet P<SB>1</SB>is passed in between the fixing roller 1 and a pressure roller 2, since passing boundary B of the small size sheet P<SB>1</SB>is set within the bending range Z of a demagnetization coil 34, the lowering of the temperature can be prevented in the passing region C of the fixing roller 1 in which the small size sheet P<SB>1</SB>is passed while preventing the rise in temperature of a non-passing region D of the fixing roller 1 in which the small size sheet P<SB>1</SB>is not passed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electromagnetic induction heating type fixing device for use in an electrophotographic image forming apparatus such as a copying machine, a laser printer, a facsimile machine, etc., for fusing and fixing (heating and fixing) an unfixed image on a sheet.

  A conventional induction heating type fixing device includes an exciting coil for induction heating a fixing roller for image formation and a degaussing coil for canceling the magnetic flux generated by the exciting coil, and an unfixed image is formed by the heated fixing roller. The unfixed image is melt-fixed on the paper carrying the toner.

  The exciting coil is arranged corresponding to a large-size paper passing area, and the degaussing coil is a non-small-sized paper that is formed on both sides of the small-sized paper passing area of the large-sized paper. It arrange | positions corresponding to a paper passing area | region (refer Unexamined-Japanese-Patent No. 2001-60490: Patent document 1 and Unexamined-Japanese-Patent No. 2001-135470: Patent document 2).

However, in the conventional induction heating type fixing device, if the distance between the end of the small-size paper passing area and the end of the demagnetizing coil is too far, the small-size paper on the fixing roller There is a drawback in that the temperature of the small-size paper in the non-sheet passing area rises on the sheet passing area side and the fixing roller is deteriorated due to excessive temperature rise. On the other hand, if the overlap between the paper passing area of the small size paper and the end of the degaussing coil is too large, the small size paper passing area in the fixing roller is near the non-paper passing area of the small size paper. There is a drawback in that the temperature is lowered, and fixing failure, gloss unevenness and offset occur in the small size paper.
JP 2001-60490 A JP 2001-135470 A

  Accordingly, an object of the present invention is to prevent the temperature increase in the non-passage area of the small size paper of the fixing roller while suppressing the temperature unevenness in the axial direction in the small size paper passage area of the fixing roller, thereby improving durability. It is an object of the present invention to provide an induction heating type fixing device that can improve the image quality and that does not cause poor fixing, uneven gloss, and offset.

In order to solve the above problems, the induction heating type fixing device of the present invention is:
The fixing roller is wound around a part of the outer periphery of the fixing roller so as to have a central extension portion substantially parallel to the axis of the fixing roller for image formation and bent portions at both ends, thereby inductively heating the fixing roller. An exciting coil,
It has a central extending portion and bent portions at both ends, one bent portion substantially overlaps with one bent portion of the exciting coil, and the extending portion is substantially the same as a part of the extending portion of the exciting coil. And a demagnetizing coil that cancels the magnetic flux generated by the exciting coil,
The boundary of the small size sheet passing region corresponding to a part of the fixing roller in the axial direction is set within the range of the other bent portion of the demagnetizing coil.

  Here, in this specification, the small size sheet means a sheet smaller than the maximum sheet that can be fixed by the fixing device. For example, the maximum sheet is A3 size paper (297 × 420 mm). In some cases, the small size sheet is A4 size paper (210 × 297 mm).

  According to the induction heating type fixing device of the present invention, the boundary of the small size sheet passing region corresponding to a part of the fixing roller in the axial direction is set within the range of the other bent portion of the demagnetizing coil. Therefore, while preventing the temperature increase in the non-passing area (non-sheet passing area) of the fixing roller through which the small size sheet does not pass, the passing area (paper passing area) of the fixing roller through which the small size sheet passes is prevented. A decrease in temperature can be prevented.

  That is, when the degaussing coil is operated, the other bent portion of the degaussing coil has a lower demagnetizing effect than the extended portion of the degaussing coil, and the demagnetizing coil is demagnetized as it goes from the other bent portion to the extending portion side. The gradient of the demagnetizing effect is such that the effect increases. Further, when the degaussing coil is not operated, the temperature immediately rises in the non-passing region in the vicinity of the passing boundary at the passing boundary (paper passing boundary) between the non-passing region and the passing region of the fixing roller. However, it has a temperature gradient such that the temperature rises from the passing boundary to the non-passing region. In the induction heating type fixing device of the present invention, since the passing boundary having the temperature gradient is set within the range of the other bent portion of the demagnetizing coil having the gradient of the demagnetizing effect, the shaft of the fixing roller The temperature distribution in the direction can be made substantially uniform.

  As described above, the temperature unevenness in the axial direction in the passing area can be suppressed while preventing the excessive temperature rise in the non-passing area, so that the durability can be improved and the fixing defect, the gloss unevenness and the offset do not occur. An induction heating type fixing device having high performance can be obtained.

  In the induction heating type fixing device of one embodiment, the number of turns of the degaussing coil is 1/2 or more of the number of turns of the exciting coil.

  According to the induction heating type fixing device of this embodiment, the demagnetizing coil can be set with a smaller number of turns than the number of turns of the exciting coil, and the degaussing coil can be reduced in size and cost.

  In the induction heating type fixing device of one embodiment, the inner surface of the extending portion of the exciting coil and the inner surface of the extending portion of the degaussing coil substantially overlap each other.

  In the induction heating type fixing device according to an embodiment, the length of the extending portion of the exciting coil is substantially equal to the width dimension (297 mm) of the A3 size paper, and the length of the extending portion of the degaussing coil is , Approximately equal to the dimension obtained by subtracting the width dimension (210 mm) of the A4 size paper from the width dimension (297 mm) of the A3 size paper.

  In the induction heating type fixing device of one embodiment, the projected shape of the demagnetizing coil on a virtual plane parallel to the axis of the fixing roller is substantially a square shape.

  According to the induction heating type fixing device of this embodiment, since one bent portion of the demagnetizing coil is substantially linear, the setting range of the passage boundary is clear and easy to set. In addition, the space of the degaussing coil can be reduced.

  Further, in the induction heating type fixing device of one embodiment, the temperature detecting means is arranged in the non-passing region of the small size sheet within the range of the other bent portion of the demagnetizing coil.

  According to the induction heating type fixing device of this embodiment, it is possible to reliably detect the temperature of the region where the temperature is expected to be highest among the regions of the fixing roller through which the small size sheet does not pass. The temperature detecting means plays a role as a safety fuse. Further, it is not necessary to arrange another temperature detecting means in the other part of the fixing roller, and the cost can be reduced.

  According to the induction heating type fixing device of the present invention, the boundary of the small size sheet passing region corresponding to a part of the fixing roller in the axial direction is set within the range of the other bent portion of the demagnetizing coil. Therefore, while preventing an increase in the temperature of the non-passing area of the fixing roller through which the small size sheet does not pass, a decrease in the temperature of the passing area of the fixing roller through which the small size sheet passes can be prevented, thereby improving durability. An induction heating type fixing device that can be improved and has good fixing performance can be realized.

  Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.

(First embodiment)
FIG. 1 is a cross-sectional configuration diagram showing an embodiment of the induction heating type fixing device of the present invention, and FIG. 2 is an enlarged cross-sectional view of a main part of the fixing roller of the present invention. As shown in FIGS. 1 and 2, the induction heating type fixing device 10 includes a fixing roller 1, a pressure roller 2 that contacts the fixing roller 1 to form a nip N, and an outer side of the fixing roller 1. And a magnetic flux generating means 3 arranged to generate a magnetic flux.

  Then, after the fixing roller 1 is heated by the magnetic flux of the magnetic flux generating means 3, the sheet P carrying the unfixed image t is passed between the fixing roller 1 and the pressure roller 2, and the nip N Thus, the unfixed image t is melt-fixed (heat-fixed) on the sheet P while the sheet P is nipped and conveyed.

  The induction heating type fixing device 10 constitutes an image forming apparatus such as a copying machine, a laser printer, or a facsimile together with an image forming means (not shown) for forming the unfixed image t on the sheet P.

  The sheet P is, for example, a recording material such as paper or an OHP sheet, and the unfixed image t is a heat-meltable toner made of, for example, a resin, a magnetic material, or a colorant.

  The fixing roller 1 and the pressure roller 2 are arranged to face each other in parallel, and both end sides thereof are rotatably supported by a bearing member (not shown). The pressure roller 2 is urged toward the shaft of the fixing roller 1 by a pressure mechanism such as a spring (not shown), and is pressed against the outer surface of the fixing roller 1 with a predetermined pressure, thereby forming the nip N.

  The fixing roller 1 is rotationally driven at a predetermined peripheral speed in a clockwise direction indicated by an arrow by a driving mechanism such as a motor (not shown). The pressure roller 2 rotates following the rotation of the fixing roller 1 by the frictional force of contact with the fixing roller 1 at the nip N.

  In the vicinity of the nip N, a temperature sensor 4 for detecting the temperature of the fixing roller 1 is installed. The magnetic flux generation means 3 is controlled by a control circuit (not shown) based on the output of the temperature sensor 4 to keep the temperature detected by the temperature sensor 4 constant.

  The fixing roller 1 includes five layers of a core metal layer 11, a heat insulating layer 12, an electromagnetic induction heat generating layer 13, an elastic layer 14, and a release layer 15 in order from the inner side to the outer side in the radial direction.

  The core metal layer 11 is for maintaining the shape of the fixing roller 1 and is, for example, a cylinder of metal such as aluminum or iron.

  The heat insulating layer 12 is for heat-insulating and holding the electromagnetic induction heat generating layer 13, and a heat-resistant and elastic rubber or resin sponge (heat insulating structure) is used. Thus, by using a heat-resistant and elastic rubber or resin sponge (heat insulating structure) for the heat insulating layer 12, the electromagnetic induction heat generating layer 13 can be reliably insulated and retained. Allowing the deflection of the induction heating layer 13 to increase the length L of the nip N and further softening the fixing roller 1 than the pressure roller 2 to improve the separation performance of the sheet P. it can.

  The electromagnetic induction heat generating layer 13 is, for example, an endless nickel electroformed belt member. As another material of the electromagnetic induction heat generating layer 13, a material having a relatively high magnetic permeability μ and an appropriate resistivity ρ such as a magnetic material (magnetic metal) such as magnetic stainless steel may be used. . In the electromagnetic induction heat generating layer 13, an eddy current is generated by the magnetic flux of the magnetic flux generating means 3 to generate heat by Joule heat, and the fixing roller 1 is heated by this heat generation. This heating is called electromagnetic induction heating.

  The elastic layer 14 is made of a rubber material or a resin material having heat resistance and elasticity, and improves the adhesion between the sheet P and the surface of the fixing roller 1. As the elastic layer 14, for example, a heat-resistant elastomer such as silicon rubber or fluororubber that can be used at a fixing temperature is used.

  The release layer 15 enhances the releasability of the surface of the fixing roller 1, and can withstand use at a fixing temperature and has toner releasability. As the release layer 15, for example, silicon rubber, fluororubber, or fluororesin such as PFA, PTFE, FEP, and PFEP is used.

  The pressure roller 2 includes three layers of a core metal layer 21, a heat insulating layer 22, and a release layer 25 in order from the inner side to the outer side in the radial direction. The core metal layer 21, the heat insulating layer 22, and the release layer 25 of the pressure roller 2 are the same as the core metal layer 11, the heat insulating layer 12, and the release layer 15 of the fixing roller 1, respectively. is there.

  A specific configuration of the fixing roller 1 will be described. The core metal layer 11 is made of iron having a thickness of 1 mm, the heat insulating layer 12 is made of Si sponge rubber having a thickness of 5 mm, and the electromagnetic induction heat generating layer 13. Is 50 μm thick, the elastic layer 14 is Si rubber having a thickness of 1 mm, the release layer 15 is PFA having a thickness of 20 μm, and the outer diameter of the fixing roller 1 is 42 mm. ing. The specific configuration of the pressure roller 2 is as follows. The core metal layer 21 is made of iron having a thickness of 1 mm, the heat insulating layer 22 is made of Si foam rubber having a thickness of 5 mm, and the mold release is performed. The layer 25 is PFA with a thickness of 30 μm, and the outer diameter of the pressure roller 2 is 38 mm.

  The magnetic flux generation means 3 is attached to the core 32, an excitation coil 31 that is attached to the core 32 and induction-heats the fixing roller 1, and a demagnetization that is attached to the core 32 and cancels the magnetic flux generated by the excitation coil 31. A coil 34 and a bobbin 33 surrounding the core 32, the excitation coil 31 and the demagnetizing coil 34 are provided, facing the fixing roller 1 on the outside of the fixing roller 1, and in the longitudinal direction of the fixing roller 1. It is arranged along.

  The core 32 is a long member having a length dimension substantially corresponding to the dimension in the axial direction (longitudinal direction) of the fixing roller 1. The core 32 has a substantially E-shaped cross section, and has a central protrusion 32a at the center in the width direction and end protrusions 32b and 32c at both ends in the width direction that extend toward the fixing roller 1 to increase heat generation efficiency. ing.

  As the core 32, for example, a high permeability and low loss material such as a ferrite core is used. The core 32 is used for increasing the efficiency of the magnetic circuit and for magnetic shielding. If there is a means that can sufficiently shield the magnetic circuit portions of the exciting coil 31 and the core 32, the protrusions 32a, 32b, and 32c may be omitted and the air core (no core) may be used. When the core 32 is made of a resin material in which magnetic powder is dispersed, the magnetic permeability is relatively low, but the shape can be freely set.

  The exciting coil 31 has a trapezoidal cross section, and has a structure in which a conducting wire is wound in the longitudinal direction along the long core 32 along a part of the outer periphery of the fixing roller 1. The central clearance of the exciting coil 31 is fitted into the central protrusion 32 a of the core 32, and the entire exciting coil 31 is within a region surrounded by the end protrusions 32 b and 32 c on both sides of the core 32.

  More specifically, as shown in the plan view of FIG. 3, the exciting coil 31 is formed by winding a conducting wire into an oval shape a plurality of times. In addition, in order to increase the current-carrying efficiency, one conducting wire is a bundle of several tens to several hundreds of strands (copper wires having a diameter of about 0.18 mm to 0.20 mm and insulated with enamel). Is a known stranded wire having a diameter of about several millimeters. In FIG. 3, for the sake of clarity, the exciting coil 31 and the demagnetizing coil 34 are shown in a developed state, and the exciting coil 31 and the demagnetizing coil 34, the fixing roller 1, and the pressure roller 2. Are shifted in the conveyance direction (arrow A direction) of the sheet P.

  The exciting coil 31 includes a pair of central extending portions 31a and 31b substantially parallel to the axis of the fixing roller 1, and a pair of bent portions 31c and 31d at both ends connecting the pair of extending portions 31a and 31b. Have The pair of extending portions 31a and 31b are substantially linear. The pair of bent portions 31 c and 31 d have a substantially arc shape and are not substantially parallel to the axis of the fixing roller 1. A gap of about several mm exists between the pair of extending portions 31a and 31b.

  The exciting coil 31 is basically tightly wound, but one extending portion 31a that is energized in the same direction is divided into an outer extending portion and an inner extending portion, and A gap of about several mm may be provided between the outer extending portion and the inner extending portion. Similarly, the other extending portion 31b energized in the same direction may be divided into an outer extending portion and an inner extending portion provided with a gap.

  A pair of extending portions 31 a and 31 b of the exciting coil 31 overlaps the fixing roller 1, and a pair of bent portions 31 c and 31 d (all or most of the exciting coil 31) is in the axial direction of the fixing roller 1. It is located outside the end of the. The length of the pair of extending portions 31a and 31b of the exciting coil 31 is substantially the same as the length in the axial direction of the fixing roller 1, and the maximum width sheet supplied to the induction heating type fixing device, in this example In this case, the value is set to a value obtained by adding a little margin to 297 mm so that A3 size paper defined by Japanese Industrial Standards can be processed. The axial length of the fixing roller 1 is slightly larger than the width dimension (297 mm) of A3 size paper.

  As shown in FIG. 1, the demagnetizing coil 34 has a trapezoidal cross section and has a structure in which a conducting wire is wound in the longitudinal direction along the long core 32. The gap is fitted into the central protrusion 32 a of the core 32, and the entire demagnetizing coil 34 is contained in a region surrounded by end protrusions 32 b and 32 c on both sides of the core 32. The demagnetizing coil 34 is sandwiched between the core 32 and the exciting coil 31. The exciting coil 31 may be sandwiched between the core 32 and the degaussing coil 34.

  More specifically, as shown in the plan view of FIG. 3, the demagnetizing coil 34 is formed by winding a conducting wire into an oval shape a plurality of times, similar to the exciting coil 31. The demagnetizing coil 34 includes a pair of central extending portions 34a and 34b extending along the axial direction of the fixing roller 1 and a pair of bent portions at both ends connecting the pair of extending portions 34a and 34b. 34c, 34d. The pair of extending portions 34a and 34b are substantially linear. The pair of bent portions 34 c and 34 d are substantially arc-shaped and are portions that are not substantially parallel to the axis of the fixing roller 1. A gap of about several mm exists between the pair of extending portions 34a and 34b.

  The demagnetizing coil 34 is basically closely wound, but one extending portion 34a that is energized in the same direction is divided into an outer extending portion and an inner extending portion. A gap of about several mm may be provided between the outer extension part and the inner extension part. Similarly, the other extending portion 34b energized in the same direction may be divided into an outer extending portion and an inner extending portion provided with a gap.

The pair of extending portions 34 a and 34 b of the degaussing coil 34 substantially overlaps part of the pair of extending portions 31 a and 31 b of the exciting coil 31, respectively. One bent portion 34 d of the degaussing coil 34 substantially overlaps with one bent portion 31 d of the exciting coil 31. A boundary B (hereinafter referred to as a passing boundary B) of the small size sheet P ₁ corresponding to a part in the axial direction of the fixing roller 1 is a range Z (hereinafter referred to as a passing boundary B) of the demagnetizing coil 34. , Which is referred to as a bent portion range Z). The The small size passage boundary of the sheet P ₁ (sheet passing boundary) B, passing area (sheet passing area) in which the small size sheet P ₁ passes through C and the non-passing area of the small size sheet P ₁ does not pass ( This is the boundary between the non-sheet passing area) D and coincides with the edge 50 of the small size sheet P ₁ .

Here, the small size sheet P ₁ means a sheet smaller than the maximum sheet that can be fixed by the induction heating type fixing device. In this embodiment, the maximum sheet is A3 size paper (297 × 420 mm), the small size sheet P ₁ is A4 size paper (210 × 297 mm).

  In other words, the pair of extending portions 34 a and 34 b of the degaussing coil 34 overlap the non-passing region D, and one bent portion 34 d (all or most of the demagnetizing coil 34) of the fixing roller 1 The other bent portion 34c (the bent portion range Z) of the degaussing coil 34 is located outside the end portion in the axial direction, and the pass region C and the non-pass region D (that is, the pass boundary B). Overlap.

  Further, the inner surfaces of the pair of extending portions 31a and 31b of the exciting coil 31 and the inner surfaces of the pair of extending portions 34a and 34b of the demagnetizing coil 34 substantially overlap, and one bent portion 31d of the exciting coil 31 is provided. And the inner surface of one bent portion 34d of the demagnetizing coil 34 substantially overlap each other. Further, the length of the pair of extending portions 34a and 34b of the demagnetizing coil 34 ranges from the width dimension (297 mm) of the A3 size paper to the width dimension (210 mm) of the A4 size paper and the bent portion range Z. The dimension of the range that overlaps with the non-passage area D is substantially equal to the subtracted dimension.

  Next, the operation of the exciting coil 31 and the degaussing coil 34 will be described.

  For example, an AC current of 10 to 100 [kHz] is applied to the excitation coil 31 by a high-frequency converter (not shown). The magnetic flux induced by the alternating current passes through the inside of the core 32 without leaking to the outside, and leaks to the outside of the core 32 for the first time between the central projection 32a and the end projections 32b and 32c of the core 32. Then, an eddy current flows through the electromagnetic induction heat generating layer 13 of the fixing roller 1 and flows through the electromagnetic induction heat generating layer 13, and the electromagnetic induction heat generating layer 13 itself generates Joule heat. The fixing roller 1 is heated by the heat generated by the electromagnetic induction heat generating layer 13.

  As shown in the circuit diagram of FIG. 4, the change-over switch 5 is attached to the demagnetizing coil 34, and the change-over switch 5 is opened and closed according to the size of the sheet P.

  That is, when passing a large size (A3 size) sheet, the demagnetizing coil 34 does not function as a coil by opening the changeover switch 5 as shown by a solid line. Therefore, it is possible to satisfactorily fix the entire area of the maximum width sheet.

On the other hand, when passing a small size (A4 size) sheet P ₁ , the demagnetizing coil 34 is guided from the exciting coil 31 by closing the changeover switch 5 as indicated by a virtual line. A counter electromotive force is generated, and a counter magnetic field is generated in a direction that hinders the change of the magnetic field, thereby exerting a demagnetizing effect. As a result, in the portion of the exciting coil 31 that overlaps the demagnetizing coil 34, the magnetic flux change caused by the exciting coil 31 is not only induced current (eddy current) in the fixing roller 1 but also reverse in the demagnetizing coil 34. An electromotive force (current associated therewith) is also generated, and the power of the magnetic field generated from the exciting coil 31 is reduced only in the portion where the demagnetizing coil 34 is present. Therefore, the temperature rise can be prevented only in the range where the demagnetizing coil 34 exists, and the temperature rise in the non-passing area of the fixing roller 1 can be suppressed.

As described above, according to the induction heating type fixing device having the above-described configuration, the induction heating type fixing device is provided without separately providing a magnetic flux generating means for a new small size sheet for the small size sheet P ₁ . The deterioration of the fixing roller 1 due to excessive temperature rise can be prevented without significantly reducing the original throughput of the apparatus, and the durability and speed of the induction heating type fixing apparatus can be increased.

Here, as shown in FIG. 3, when the demagnetizing coil 34 is operated, the bending portion range Z of the demagnetizing coil 34 has a lower demagnetizing effect than the extending portions 34a and 34b of the demagnetizing coil 34, and the other The effect gradient is such that the demagnetizing effect increases from the bent portion 34c toward the extending portions 34a and 34b. Further, when the demagnetizing coil 34 is not operated, the temperature does not immediately rise in the non-passage region D near the passage boundary B even at the passage boundary B of the small size sheet P _1. The temperature gradient is such that the degree of temperature rise increases from B to the non-passage region D.

  In the present invention, the temperature distribution in the axial direction of the fixing roller 1 is substantially uniform by disposing the passing region B having a temperature gradient in the bent portion range Z of the demagnetizing coil 34 having a gradient of the demagnetizing effect. To make it possible.

Next, FIG 5A~ Figure 5C, the fixing of the time of changing the position with respect to the fixing roller 1 (the exciting coil 31 and the degaussing coil 34) of the passage boundary B of the small size sheet P ₁ shown in FIG. 3 The temperature distribution in the axial direction of the roller 1 is shown. The horizontal axis shows the measurement position of the fixing roller 1 in the axial direction, and the vertical axis shows the temperature of the fixing roller 1. The horizontal axes (III) to (IV) correspond to the bent portion range Z of the degaussing coil 34 shown in FIG. In the figure, the interval between the paper passing area and the non-paper passing area corresponds to the passage boundary B.

In FIG. 5A, as shown in the embodiment, the passage boundary B of the small size sheet P ₁ is set within the bent portion range Z (horizontal axes (III) to (IV)) of the demagnetizing coil 34. The axial temperature distribution of the fixing roller 1 is shown.

  As can be seen from FIG. 5A, the temperature in the non-sheet-passing area does not increase with time and the temperature in the non-sheet-passing area does not decrease. Unevenness can be suppressed. In this way, it is possible to obtain an induction heating type fixing device that can improve durability and has high performance without occurrence of poor fixing, uneven gloss, and offset.

On the other hand, FIG. 5B shows the passage boundary B of the small-size sheet P ₁ , extending portions 34 a and 34 b of the degaussing coil 34 outside the bent portion range Z of the degaussing coil 34 (horizontal axes (II) to (III The temperature distribution in the axial direction of the fixing roller 1 when set to)) is shown.

  As can be seen from FIG. 5B, the temperature rise in the non-sheet passing area is suppressed with time, but the temperature drop in the sheet passing area (horizontal axes (III) to (IV)) occurs. Fixing defects, uneven gloss, offset, etc. may occur.

Further, FIG. 5C shows the passage boundary B of the small size sheet P ₁ in the non-installation area (horizontal axes (IV) to (V)) of the degaussing coil 34 outside the bent portion range Z of the degaussing coil 34. The axial temperature distribution of the fixing roller 1 when set is shown.

  As can be seen from FIG. 5C, with the passage of time, there is no temperature drop in the sheet passing area, but a temperature increase has occurred in the non-sheet passing area (horizontal axis (IV)). Deterioration due to excessive temperature rise may occur.

Thus, as can be seen from FIGS. 5A to 5C, the passage boundary B of the small size sheet P ₁ is set within the bent portion range Z (horizontal axes (III) to (IV)) of the demagnetizing coil 34. In this case, the temperature unevenness in the axial direction in the sheet passing area can be suppressed while preventing an excessive temperature rise in the non-sheet passing area.

  Next, FIG. 6 shows changes in the end temperature of the non-passing area (non-paper passing area) of the fixing roller 1 when the degaussing coil 34 is not installed and when the number of turns of the degaussing coil 34 is changed. Show. The elapsed time is shown on the horizontal axis, and the temperature of the fixing roller 1 is shown on the vertical axis. When changing the number of turns of the degaussing coil 34, the number of turns of the exciting coil 31 is 10, whereas the number of turns of the degaussing coil 34 is changed to 4, 6, and 8. Yes.

  As can be seen from FIG. 6, when the demagnetizing coil 34 is not installed, the end temperature of the fixing roller 1 increases with time. In the case where the demagnetizing coil 34 is installed, when the number of turns of the degaussing coil 34 with respect to the number of turns of the excitation coil 31 is less than ½ (the number of turns of the degaussing coil 34 is 4), The end temperature of the fixing roller 1 tends to slightly increase with time, but the number of turns of the degaussing coil 34 with respect to the number of turns of the exciting coil 31 is 1/2 or more (the number of turns of the degaussing coil 34 is (6 times and 8 times), the increase in the end temperature of the fixing roller 1 is suppressed.

  Thus, the number of turns of the degaussing coil 34 is preferably ½ or more of the number of turns of the excitation coil 31, and the degaussing coil 34 is configured with a smaller number of turns than the number of turns of the excitation coil 31. The demagnetizing coil 34 can be reduced in size and cost.

(Second Embodiment)
FIG. 7 shows another embodiment of the present invention. A pair of bent portions 34 e and 34 f of the degaussing coil 34 are substantially perpendicular to the axis of the fixing roller 1. In other words, the projected shape of the degaussing coil 34 on a virtual plane parallel to the axis of the fixing roller 1 along the circumferential direction of the fixing roller 1 is a substantially square shape. Since other structures are the same as those of the first embodiment, description thereof is omitted.

  According to the induction heating type fixing device having this configuration, the other bent portion 34e of the demagnetizing coil 34 is substantially linear, so that the bent portion range Z of the demagnetizing coil 34 becomes clear, and the passing boundary B It becomes easy to set. Further, the space of the degaussing coil 34 is reduced.

(Third embodiment)
As another embodiment of the present invention, as shown by an imaginary line in FIG. 3, a temperature sensor or the like is provided in the non-passing region D of the small size sheet P ₁ within the range Z of the other bent portion 34 c of the demagnetizing coil 34. The temperature detecting means 6 is arranged.

According to the induction heating type fixing device of this configuration, when passing between the small-size sheet P ₁ between the fixing roller 1 and the pressure roller 2, the fixing roller 1 where the small-sized sheet P ₁ does not pass In this region, the temperature of the region where the temperature is expected to be highest can be reliably confirmed, and the temperature detecting means 6 plays a role as a safety fuse. Further, it is not necessary to arrange another temperature detecting means in the other part of the fixing roller 1, and the cost can be reduced.

  That is, in the demagnetizing coil 34, the other bent portion 34c has a lower demagnetizing effect than the pair of extending portions 34a and 34b, so that the temperature of the fixing roller 1 overlaps with the other bent portion 34c. By arranging the detection means 6 and confirming the degree of temperature rise in the non-passing area, it is not necessary to arrange another temperature detection means in another area of the demagnetizing coil 34, and cost reduction is realized. it can.

  In addition, this invention is not limited to the above-mentioned embodiment. For example, although not shown, a driving means for moving the demagnetizing coil 34 in the axial direction of the fixing roller 1 is separately provided, and the demagnetizing coil 34 is moved by the driving means, so that the bending portion range Z of the demagnetizing coil 34 is set. May be superposed on the passage boundary B of the sheets P of various sizes, and an induction heating type fixing device that can cope with the sheets P of various sizes can be realized.

1 is a cross-sectional configuration diagram illustrating an embodiment of an induction heating type fixing device of the present invention. FIG. 3 is an enlarged cross-sectional view of a main part showing a developed state of the fixing roller. It is explanatory drawing which shows the positional relationship of an exciting coil, a degaussing coil, a fixing roller, a pressure roller, and a sheet | seat. It is a circuit diagram which shows the switch circuit of a degaussing coil. FIG. 6 is a first graph showing the measurement position of the fixing roller on the horizontal axis and the temperature of the fixing roller on the vertical axis. FIG. 6 is a second graph showing the measurement position of the fixing roller on the horizontal axis and the temperature of the fixing roller on the vertical axis. FIG. 6 is a third graph showing the measurement position of the fixing roller on the horizontal axis and the temperature of the fixing roller on the vertical axis. FIG. 6 is a graph showing elapsed time on the horizontal axis and the temperature of the fixing roller on the vertical axis. It is a simplified top view which shows other embodiment of a degaussing coil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fixing roller 2 Pressure roller 3 Magnetic flux generation means 6 Temperature detection means 10 Induction heating type fixing device 31 Excitation coil 31a, 31b Extension part 31c, 31d Bending part 34 Demagnetization coil 34a, 34b Extension part 34c, 34d, 34e, 34f Bent part P ₁ Small size sheet B Passing boundary C Passing area D Non-passing area Z Bending part range

Claims (4)

The fixing roller is wound around a part of the outer periphery of the fixing roller so as to have a central extension portion substantially parallel to the axis of the fixing roller for image formation and bent portions at both ends, thereby inductively heating the fixing roller An exciting coil to
It has a central extending portion and bent portions at both ends, one bent portion substantially overlaps with one bent portion of the exciting coil, and the extending portion is substantially part of the extending portion of the exciting coil. And a degaussing coil that cancels the magnetic flux generated by the excitation coil,
An induction heating type fixing device, wherein a boundary of a small size sheet passing region corresponding to a part of the fixing roller in an axial direction is set within a range of the other bent portion of the demagnetizing coil. The induction heating type fixing device according to claim 1,
The induction heating type fixing device, wherein the number of turns of the degaussing coil is ½ or more of the number of turns of the exciting coil. The induction heating type fixing device according to claim 1,
An induction heating type fixing device, wherein a projected shape of the degaussing coil onto a virtual plane parallel to the axis of the fixing roller is a substantially square shape. The induction heating type fixing device according to claim 1,
An induction heating type fixing device, wherein a temperature detecting means is arranged in a non-passing region of the small size sheet within the range of the other bent portion of the demagnetizing coil.

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