Heating device, fixing device and image forming apparatus

Abstract

To stably cut off energization to a heating member when the heating member runs out of control.SOLUTION: A heating device has a heater 22 as a heating member and a safety device 25 having a thermo-sensitive surface that detects the temperature of the heater 22, and when the temperature of the thermo-sensitive surface becomes equal to or more than a predetermined temperature, stops supply of power to the heater 22. The heating device has a holding member 23 that holds the heater 22 from the back side. The holding member 23 has a through hole 23d continuing to the back side of the heater 22, accommodates the safety device 25 in the through hole 23d, forms step parts 23k that hold an outer peripheral part of the thermo-sensitive surface on the inner periphery of the through hole 23d, and maintains, with the step parts 23k, the non-contact state between the heater 22 and a center part of the thermo-sensitive surface of the safety device 25.SELECTED DRAWING: Figure 5B

Description

[Technical field]

[0001] The present invention relates to a heating device, a fixing device, and an image forming apparatus, and more particularly to a heating device having a safety device for preventing overheating of a heating member, and a fixing device and an image forming apparatus equipped with the heating device. [Background technology]

[0002] Image forming devices such as copiers and printers are equipped with heating devices having heating members, such as a fixing device that fixes toner on paper by heat, a drying device that dries ink on paper, etc. Such heating devices are provided with safety devices such as thermostats to protect the power supply circuit and prevent overheating of the heating members.

[0003] Such a safety device is usually arranged with its entire heat-sensitive surface in contact with the heating member in order to improve responsiveness. However, the safety device generally has a relatively large heat capacity. Therefore, if the entire heat-sensitive surface of the safety device directly contacts the heating member, the temperature of the heating member in the contact area will locally drop, which is likely to cause uneven heating of the image. In addition, the safety device is likely to malfunction when the temperature of the heating member slightly exceeds the set value due to overshoot.

[0004] In the invention of Patent Document 1 (Japanese Patent No. 4546233), as shown in Fig. 11A, a spacer 36 is interposed between a safety device 25 and a heater 22, which is a heating member. As shown in spacers 36-1 to 36-4 in Fig. 11B, the spacer 36 has an opening, a frame, and legs extending inward from the frame.

[0005] The opening reduces the contact area between the spacer 36 and the heater 22. If the heater 22 experiences an abnormal temperature rise, the spacer 36 softens and melts. When the safety device 25 comes into contact with the heater 22 through the melted leg, the safety device 25 is activated and cuts off the power supply to the heater 22. Summary of the Invention [Problem to be solved by the invention]

[0006] However, the invention of Patent Document 1 has a configuration in which the legs of the spacer are interposed as separate parts between the center of the heat-sensitive surface of the safety device and the heating member. Therefore, due to variations in the part shapes of both the safety device and the spacer, when the spacer 36 is in a non-melted state as shown in Fig. 11C(a), the contact state between the safety device 25 and the spacer legs and the contact state between the spacer legs and the heater 22 vary. If there is such variation in the contact state, the time until the spacer legs start to melt as shown in Fig. 11C(b) when the heater 22 goes out of control varies, and the current to the heater 22 cannot be cut off stably.

[0007] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a heating device that can stably cut off the power supply to a heating member when the heating member goes out of control. [Means for solving the problem]

[0008] In order to solve the above problem, the heating device of the present invention has a heating element and a safety device having a heat-sensitive surface that detects the temperature of the heating element, and stops supplying power to the heating element when the temperature of the heat-sensitive surface reaches or exceeds a predetermined temperature.The heating device has a holding member that holds the heating element from the back side, the holding member has a through hole that leads to the back side of the heating element, the safety device is accommodated in the through hole, and a step portion that holds the outer periphery of the heat-sensitive surface is formed on the inner circumference of the through hole, and the step portion maintains the center of the heat-sensitive surface of the safety device and the heating element in non-contact. Effect of the Invention

[0009] According to the present invention, it is possible to stably cut off the current supply to the heating member when the heating member goes out of control. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2A]FIG. 2 is a schematic configuration diagram of a fixing device according to the present embodiment. [Figure 2B] FIG. 2 is a perspective view of the fixing device excluding a fixing belt, a stay, and the like. [Figure 2C] FIG. 4 is a perspective view showing a mounting structure of a heater to a holding member of the fixing device. [Figure 2D] FIG. 2 is an exploded perspective view of a heater, a holding member, a stay, etc. [Figure 3A] FIG. [Figure 3B] 13A and 13B are diagrams showing a state in which a connector is attached to a holding member. [Figure 4A] FIG. [Figure 4B] FIG. 4 is a side view showing the mounting structure of the safety device. [Figure 4C] FIG. 4 is a plan view showing the mounting structure of the safety device. [Figure 4D] FIG. 11 is a side view showing another mounting structure of the safety device. [Figure 5A] 4C , (a) is a xx cross-sectional view, (b) is a yy cross-sectional view, and (c) is a zz cross-sectional view. [Figure 5B] 1A to 1D are cross-sectional views showing a mounting structure of a safety device to a holding member, in which (a) is a cross-sectional view of a first embodiment, (b) is a cross-sectional view of a second embodiment, (c) is a cross-sectional view of a third embodiment, and (d) is a cross-sectional view of a fourth embodiment. [Figure 6A] FIG. 13A is a plan view showing an example in which a protruding engagement portion of a safety device is protruded in the width direction, and FIG. [Figure 6B] 13 is a plan view showing another example in which a convex engagement portion of a safety device protrudes in the width direction. FIG. [Figure 6C] 13 is a plan view showing an example in which a protruding engagement portion of a safety device protrudes in the longitudinal direction. FIG. [Figure 6D] 13 is a plan view showing an example in which concave engagement portions of a safety device are formed on both sides in the width direction. FIG. [Figure 6E] 13A and 13B are a plan view and a cross-sectional view showing an example in which a protruding engagement portion of a safety device is protruded toward a heater in a thickness direction. [Figure 7A] FIG. 11 is a cross-sectional view showing another type 1 of the stay. [Figure 7B]FIG. 11 is a cross-sectional view showing another type 2 of the stay. [Figure 7C] FIG. 11 is a cross-sectional view showing another type 3 of the stay. [Figure 8] FIG. 1A is a plan view showing the coaxial arrangement of a temperature detection element and a point of application of a coil spring, and FIG. [Figure 9A] FIG. 13 is a diagram showing the configuration of another fixing device. [Figure 9B] FIG. 13 is a diagram showing the configuration of another fixing device. [Figure 9C] FIG. 13 is a diagram showing the configuration of still another fixing device. [Figure 10] 1A and 1B are cross-sectional views showing a first configuration and a second configuration, respectively, of a conventional structure for mounting a safety device to a holding member. [Figure 11A] 1 is a cross-sectional view showing a conventional mounting structure of a safety device to a holding member. [Figure 11B] 1A to 1D are plan views showing different types of spacers. [Figure 11C] 13(a) and 13(b) are cross-sectional views showing the movement of the safety device when a spacer between the retaining member and the safety device softens. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention will be described below with reference to the accompanying drawings. In each drawing for explaining the present invention, components such as members and components having the same function or shape are given the same reference numerals as far as they can be distinguished, and the description thereof will be omitted after the first description.

[0012] (● Overview of image forming device) 1 is a schematic diagram of an image forming apparatus 100 according to an embodiment of the present invention. Note that the image forming apparatus 100 may be a printer, a copier, a facsimile, or a combination machine thereof.

[0013] The image forming apparatus 100 shown in Fig. 1 includes four imaging units 1Y, 1M, 1C, and 1Bk that are image forming sections. Each imaging unit 1Y, 1M, 1C, and 1Bk is configured to be detachable from the image forming apparatus main body 103, and has the same configuration except that it contains a developer of a different color, yellow, magenta, cyan, or black, corresponding to the color separation components of a color image. Specifically, each imaging unit 1Y, 1M, 1C, and 1Bk includes a drum-shaped photoconductor 2 as an image carrier, a charging device 3 that charges the surface of the photoconductor 2, a developing device 4 that supplies toner as a developer to the surface of the photoconductor 2 to form a toner image, and a cleaning device 5 that cleans the surface of the photoconductor 2.

[0014] The image forming apparatus 100 also includes an exposure device 6 that exposes the surface of each photoconductor 2 to form an electrostatic latent image, a paper feeding device 7 that supplies paper P as a recording medium, a transfer device 8 that transfers the toner image formed on each photoconductor 2 to the paper P, a fixing device 9 that fixes the toner image transferred to the paper P, and a paper discharge device 10 that discharges the paper P outside the apparatus.

[0015] The transfer device 8 has an endless intermediate transfer belt 11 as an intermediate transfer body stretched by multiple rollers, four primary transfer rollers 12 as primary transfer members that transfer the toner images on the photoconductors 2 to the intermediate transfer belt 11, and a secondary transfer roller 13 as a secondary transfer member that transfers the toner images transferred onto the intermediate transfer belt 11 to paper P. Each of the multiple primary transfer rollers 12 is in contact with the photoconductor 2 via the intermediate transfer belt 11.

[0016] As a result, the intermediate transfer belt 11 and each photoconductor 2 come into contact with each other, forming a primary transfer nip between them. Meanwhile, the secondary transfer roller 13 comes into contact with one of the rollers that stretch the intermediate transfer belt 11 via the intermediate transfer belt 11. As a result, a secondary transfer nip is formed between the secondary transfer roller 13 and the intermediate transfer belt 11.

[0017] Further, a paper transport path 14 along which the paper P sent out from the paper feeder 7 is transported is formed within the image forming apparatus 100. A pair of timing rollers 15 is provided on the paper transport path 14 midway from the paper feeder 7 to the secondary transfer nip (secondary transfer roller 13).

[0018] (● Printing operation of image forming device) Next, the printing operation of the image forming apparatus will be described with reference to Fig. 1. When an instruction to start a printing operation is given, in each of the imaging units 1Y, 1M, 1C, and 1Bk, the photoconductor 2 is rotated clockwise in Fig. 1, and the surface of the photoconductor 2 is uniformly charged to a high potential by the charging device 3.

[0019] Next, based on the image information of the document read by the document reader or the print information instructed to be printed from the terminal, the exposure device 6 exposes the surface of each photoconductor 2 to light, lowering the potential of the exposed portion and forming an electrostatic latent image. Then, toner is supplied from the developing device 4 to this electrostatic latent image, and a toner image is formed on each photoconductor 2.

[0020] When the toner images formed on each photoconductor 2 reach the primary transfer nip (position of primary transfer roller 12) as each photoconductor 2 rotates, the toner images are transferred to the intermediate transfer belt 11, which rotates counterclockwise in Fig. 1, so as to be overlapped one after the other. The toner images transferred onto the intermediate transfer belt 11 are then transported to the secondary transfer nip (position of secondary transfer roller 13) as the intermediate transfer belt 11 rotates, and are transferred to the paper P that has been transported at the secondary transfer nip.

[0021] This paper P is supplied from the paper feeder 7. The paper P supplied from the paper feeder 7 is temporarily stopped by a timing roller 15, and then conveyed to the secondary transfer nip in accordance with the timing at which the toner image on the intermediate transfer belt 11 reaches the secondary transfer nip.

[0022] In this way, a full-color toner image is carried on the paper P. After the toner image is transferred, the toner remaining on each photoconductor 2 is removed by each cleaning device 5. Note that a direct transfer method is also possible in which the toner image is directly transferred to the paper P by passing the paper P sequentially through the primary transfer nip without using the intermediate transfer belt 11.

[0023] The paper P onto which the toner image has been transferred is transported to a fixing device 9, which fixes the toner image onto the paper P. Thereafter, the paper P is discharged outside the apparatus by a paper discharge device 10, completing a series of printing operations.

[0024] (● Fixing device) Next, a description will be given of the configuration of the fixing device 9. As shown in Fig. 2A, the fixing device 9 according to this embodiment includes a fixing belt 20 made of an endless belt member as a fixing member, a pressure roller 21 as an opposing member disposed opposite to the outer circumferential surface of the fixing belt 20, and a planar heater 22 as a heating member for heating the fixing belt 20.

[0025] A holding member 23 and a stay 24 that hold the heater 22 are disposed on the inner peripheral side of the fixing belt 20. The holding member 23 is reinforced and supported in the longitudinal direction by the stay 24, which is a reinforcing member.

[0026] 2B, a plurality of safety devices 25, 26, 27, 55 are disposed at a plurality of positions in the longitudinal direction on the rear side of heater 22. The rear side of heater 22 is held by holding member 23 as shown in FIGS. 2C and 2D.

[0027] 2C, a plurality of heat generating blocks 59 are disposed on the front side of the heater 22. Details of the safety devices 25, 26, 27, and 55 will be described later with reference to FIGS.

[0028] The stay 24 described above is made of a metal channel material, and both ends thereof are supported on both side walls of the fixing device 9 via cap members 24e shown in Figures 2B and 2D. The stay 24 supports the surface of the holding member 23 opposite to the heater 22 side, so that the heater 22 and the holding member 23 are kept linear without being significantly deflected by the pressure force of the pressure roller 21. As a result, a nip portion N is formed between the fixing belt 20 and the pressure roller 21, which maintains a constant pressure in the width direction.

[0029] The holding member 23 is desirably formed of a heat-resistant material because it is likely to become hot due to the heat of the heater 22. For example, when the holding member 23 is formed of a heat-resistant resin with low thermal conductivity such as LCP or PEEK, the heat transfer from the heater 22 to the holding member 23 is suppressed, and the fixing belt 20 can be heated efficiently.

[0030] The heat-resistant resin can be selected from LCP resin, phenol resin, fluororesin, polyimide resin, polyamide resin, polyamideimide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, and FEP resin. When the holding member 23 is formed from these heat-resistant resins, the heat-resistant resin can be extruded in the holder longitudinal direction to form an extrusion molded product.

[0031] The fixing belt 20 has a tubular body made of polyimide (PI) having an outer diameter of 24 mm and a thickness of 40 to 120 μm, for example. A release layer made of a fluorine-based resin such as PFA or PTFE and having a thickness of 5 to 50 μm is formed on the outermost layer of the fixing belt 20 in order to increase durability and ensure releasability.

[0032] An elastic layer made of rubber or the like having a thickness of 50 to 500 μm may be provided between the substrate and the release layer. The substrate of the fixing belt 20 is not limited to polyimide, and may be a heat-resistant resin such as PEEK or a metal substrate such as nickel (Ni) or SUS. The inner peripheral surface of the fixing belt 20 may be coated with polyimide, PTFE, or the like as a sliding layer.

[0033] The pressure roller 21 has an outer diameter of, for example, 24 mm to 30 mm, and is composed of a solid iron core 21a, an elastic layer 21b formed on the surface of the core 21a, and a release layer 21c formed on the outside of the elastic layer 21b. The elastic layer 21b is made of silicone rubber and has a thickness of, for example, 3 mm to 4 mm.

[0034] In order to improve releasability, it is preferable to form a release layer 21c made of a fluororesin layer having a thickness of, for example, about 40 μm on the surface of the elastic layer 21b. As a member facing the outer circumferential surface of the fixing belt 20, a member such as an endless pressure belt can be used instead of the pressure roller 21.

[0035] The heater 22 is provided longitudinally across the width of the fixing belt 20, and is disposed so as to be in contact with the inner circumferential surface of the fixing belt 20. The heater 22 may be in non-contact with the fixing belt 20 or indirectly in contact with the fixing belt 20 via a low-friction sheet or the like, but having the heater 22 in direct contact with the fixing belt 20 improves the efficiency of heat transfer to the fixing belt 20.

[0036] The heater 22 can also be in contact with the outer circumferential surface of the fixing belt 20. However, in this case, there is a risk that the fixing quality will decrease if the outer circumferential surface of the fixing belt 20 is damaged by contact with the heater 22. For this reason, it is preferable to have the heater 22 in contact with the inner circumferential surface of the fixing belt 20.

[0037] The pressure roller 21 and the fixing belt 20 are pressed against each other by a spring as a pressure means, whereby a nip portion N is formed between the fixing belt 20 and the pressure roller 21. The pressure roller 21 also functions as a drive roller that is rotated by a driving force transmitted from a driving means provided in the image forming apparatus main body 103.

[0038] On the other hand, the fixing belt 20 is configured to rotate following the rotation of the pressure roller 21. Since the fixing belt 20 slides against the heater 22 during rotation, a lubricant such as oil or grease may be interposed between the heater 22 and the fixing belt 20 to improve the sliding property of the fixing belt 20.

[0039] When the printing operation is started, the pressure roller 21 is rotated and the fixing belt 20 starts to rotate. Furthermore, the fixing belt 20 is heated by supplying power to the heater 22. Then, when the temperature of the fixing belt 20 reaches a predetermined target temperature (fixing temperature), as shown in FIG. 2A, the paper P carrying an unfixed toner image is conveyed between the fixing belt 20 and the pressure roller 21 (nip portion N), whereby the unfixed toner image is heated and pressurized to be fixed to the paper P.

[0040] (● Heater configuration) Fig. 3A is a plan view of the heater 22, and Fig. 3B is a perspective view showing a state in which a connector 70 is attached to the heater 22 and the holding member 23. The heater 22 has a base material 50, a heat insulating layer, and a coating layer. The heater 22 is formed in a roughly rectangular plate shape with the axial direction of the fixing belt 20 as the longitudinal direction. The width of the heater 22 in the paper transport direction is, for example, 13 mm.

[0041] As shown in Fig. 3A, three heat generating parts 60B, 60A, 60B are provided along the longitudinal direction on the base material 50 of the heater 22. The center of the base material 50 in the longitudinal direction is the central heat generating part 60A as a first heat generating part, and the remaining two are end heat generating parts 60B as second heat generating parts arranged on both sides of the central heat generating part 60A in the longitudinal direction.

[0042] A plurality of electrode portions 61 for supplying power to each heat generating portion are disposed on one end of the substrate 50. Here, three electrode portions 61, a first electrode portion 61A, a second electrode portion 61B, and a third electrode portion 61C, are disposed in this order from the right in FIG. 3A.

[0043] Each of the heat generating portions 60A and 60B is composed of a plurality of heat generating blocks 59 connected in parallel. All the heat generating blocks 59 are connected to the third electrode portion 61C by a common power supply line 62A.

[0044] Meanwhile, the heat generating block 59 of the central heat generating portion 60A and the first electrode portion 61A are connected by a power supply line 62B, and the heat generating blocks 59 of the end heat generating portions 60B at both ends are connected to the second electrode portion 61B by power supply lines 62C, 62D. By connecting the central heat generating portion 60A and the end heat generating portions 60B in parallel to the electrodes 61A to 61C in this manner, the heat generation can be controlled independently of each other.

[0045] 3A is the widthwise passing area when paper P1 having a width smaller than the width L1 of the central heat generating portion 60A passes through the nip portion N. The paper passing area indicated by W2 in the figure is the widthwise passing area when paper P2 having a width larger than the width L1 of the central heat generating portion 60A passes through the nip portion N.

[0046] When the width of the paper being passed through is equal to or less than the width L1 of the central heating part 60A in Fig. 3A, only the central heating part 60A is heated. When the width of the paper being passed through is greater than the width L1 of the central heating part 60A, in addition to the central heating part 60A, each of the end heating parts 60B is heated. In this way, the size of the heating area can be changed according to the size of the paper passing area.

[0047] Furthermore, the width L1 of the central heat generating portion 60A is adjusted to the width of small-sized paper (e.g., A4 paper width: 215 mm), and the width L2 of the heat generating region including one end heat generating portion 60B and the other end heat generating portion 60B is adjusted to the width of large-sized paper (e.g., A3 paper width: 301 mm). This limits the size of the non-paper passing region when these papers are passed through, making it difficult for excessive temperature rise to occur. This can therefore increase printing productivity.

[0048] 3B, a connector 70 is inserted and connected to the three electrode portions 61 from the short side direction of the holding member 23. The connector 70 has a resin housing 71 and a leaf spring contact terminal 72 fixed to the housing 71.

[0049] The contact terminal 72 has three contact points 72a which come into contact with the electrode portions 61 of the heater 22. In addition, a harness (conductor wire) 73 for power supply is connected to the connector 70 (contact terminal 72).

[0050] The connector 70 is attached so as to sandwich the heater 22 and the holding member 23 from the front and back sides together. This causes each contact portion 72a of the contact terminal 72 to elastically contact (pressure weld) with the electrode portion 61 of the heater 22. In this way, the central heating portion 60A, the end heating portion 60B and the power source provided in the image forming apparatus are electrically connected via the connector 70, and the power source becomes capable of supplying power to the central heating portion 60A and the end heating portion 60B.

[0051] Furthermore, as in this embodiment, the connector 70 serving as a power supply member also functions as a clamping member that holds the heater 22 and the holding member 23 together, eliminating the need to provide a separate clamping member and making it possible to reduce the number of parts.

[0052] (● Placement of safety devices) Next, the arrangement of the above-mentioned safety devices 25 to 27, 55 will be described with reference to Fig. 3A. Hereinafter, as necessary, they will be referred to as a first safety device 25, a second safety device 26, a third safety device 27, and a fourth safety device 55. These safety devices may be constituted by, for example, a thermocut, a thermostat, a thermistor, or the like.

[0053] Thermocut is a one-shot type protective element with a thermal cutoff function. When the heater temperature exceeds the threshold, the thermocut cuts off the power supply to the heater so that it cannot be restored. The thermostat cuts off the power supply to the heater when the heater temperature exceeds the threshold, and restores the power supply to the heater when the heater temperature falls below the threshold.

[0054] The thermistor is a semiconductor element that measures the heater temperature. Image forming apparatus 100 controls the heater based on the temperature measured by the thermistor. Here, first to third safety devices 25-27 are configured with thermistors, and fourth safety device 55 is configured with a thermostat.

[0055] The temperature detection unit 25a of the first safety device 25 is disposed within the width L1 of the central heat generating portion 60A and further within the small size paper passing region W1. In this manner, the temperature detection unit 25a of the first safety device 25 in Fig. 3A is disposed within the width L1 of the central heat generating portion 60A and further within the small size paper passing region W1, so that the first safety device 25 can detect the temperature of the paper passing region in the central heat generating portion 60A when small size paper P1 or papers of widths larger than small size paper P1 are passed through.

[0056] In addition, when there are multiple types of paper with width sizes smaller than the width L1 of the central heating portion 60A, by positioning the temperature detection portion 25a of the first safety device 25 within the paper passage area of ​​the smallest width paper, the first safety device 25 can detect the temperature of paper passage areas of all sizes passing over the central heating portion 60A.

[0057] The temperature detection unit 26a of the second safety device 26 is disposed outside the width L1 of the central heat generating unit 60A and within the large size paper passing area W2. That is, the temperature detection unit 26a of the second safety device 26 is disposed in correspondence with the paper passing area where the large size paper P2 passes over the end heat generating unit 60B when the large size paper P2 is passed.

[0058] In this way, since the temperature detection section 26a of the second safety device 26 is positioned outside the width L1 of the central heating section 60A and within the large size paper passing area W2, the second safety device 26 can detect the temperature of the paper passing area at the end heating section 60B when a large size paper P2 is passed through.

[0059] Furthermore, when there are multiple types of paper that pass over the edge heating portion 60B, the temperature detection portion 26a of the second safety device 26 is disposed in the paper passage area for the smallest width paper among them. This allows the second safety device 26 to detect the temperature of the paper passage area for all sizes that pass over the edge heating portion 60B.

[0060] The temperature detection unit 27a of the third safety device 27 is disposed outside the small size paper passing area W1 and within the width L1 of the central heat generating part 60A. That is, the temperature detection unit 27a of the third safety device 27 is disposed in correspondence with a non-paper passing area (non-passing area) where the small size paper P1 does not pass over the central heat generating part 60A when the small size paper P1 is passed.

[0061] In this way, since the temperature detection section 27a of the third safety device 27 is positioned outside the small-size paper passing area W1 and within the width L1 of the central heating section 60A, the third safety device 27 can detect the temperature of the non-paper passing area in the central heating section 60A when a small-size paper P1 is passed through.

[0062] The temperature detection unit 55a of the fourth safety device 55 is disposed at approximately the center of the width L1 of the central heat generating unit 60A. When the heater 22 goes out of control and the temperature of the central heat generating unit 60A becomes equal to or higher than a threshold value, the temperature detection unit 55a detects the temperature and cuts off the power supply to the heater 22. When the temperature of the heater 22 (the central heat generating unit 60A) falls below the threshold value, the temperature detection unit 55a detects the temperature drop and restores the power supply to the heater 22.

[0063] The fourth safety device 55 is disposed adjacent to the first safety device 25. In this way, by disposing the fourth safety device 55 and the first safety device 25 adjacent to each other, even if the fourth safety device 55 is unable to detect an excessive rise in temperature, the first safety device 25 can detect an abnormal drop in temperature caused by a broken wire, thereby making it possible to grasp a malfunction of the heater 22. Also, the fourth safety device 55 can be a fuse instead of a thermostat.

[0064] The temperature information detected by each of the safety devices 25, 26, and 27 is sent to a control unit that controls the heat generation of each of the heat generating units 60A and 60B, and each of the heat generating units 60A and 60B is individually controlled based on the sent temperature information. In this way, the temperature of the nip portion N is controlled to a preset target temperature (fixing temperature).

[0065] When small sized sheets of paper are continuously fed, the temperature may rise excessively if the heat of the heater 22 in the non-paper passing area is not consumed much. In such a case, the third safety device 27 detects that the temperature in the non-paper passing area has reached a predetermined temperature or higher, and controls the heater 22 to reduce the amount of heat generated. Furthermore, the temperature rise in the non-paper passing area is suppressed by slowing down the paper conveying speed, widening the paper conveying interval, or stopping image formation.

[0066] In this embodiment, the second safety device 26 is disposed only on the side of one end heat generating portion 60B, but the second safety device 26 may also be disposed on the side of the other end heat generating portion 60B. However, in the case of an image forming apparatus using a so-called center conveyance reference method in which the sheets P1 and P2 of different sizes are conveyed with their widthwise center positions M aligned as in this embodiment, the temperature distribution of the fixing belt is basically symmetrical with respect to the widthwise center position M of the sheet. Therefore, if the second safety device 26 is disposed only on the side of one end heat generating portion 60B, the other end heat generating portion 60B can also be controlled in the same way.

[0067] In each of the above-mentioned embodiments, the heater has a plurality of heat generating parts (central heat generating part 60A and end heat generating part 60B) that are controlled independently of each other, but the present invention is not limited to heaters having a plurality of heat generating parts, and can also be applied to heaters having only one heat generating part. Also, in the above-mentioned embodiments, the holding member 23 in which the safety devices 25-27, 55 are positioned is used as the mating member, but the mating member is not limited to this and may be the stay 24 or another member.

[0068] (● Safety device configuration) Next, there will be described a configuration of each of the safety devices 25 to 27, 55. Since each of the safety devices 25 to 27, 55 can be configured in the same manner, the configuration of one of the safety devices 25 will be described.

[0069] Fig. 4A is a plan view of safety device 25, Fig. 4B is a side view of safety device 25 attached to holding member 23, and Fig. 4C is a plan view of the same. Safety device 25 includes a temperature detection element 31 functioning as temperature detection section 25a, a holder 32 that holds temperature detection element 31, a buffer member 33 provided between temperature detection element 31 and holder 32, an insulating sheet 34 that holistically covers temperature detection element 31 together with holder 32, and two lead wires 35 that are conductors electrically connected to temperature detection element 31.

[0070] The holder 32 is made of a resin material such as LCP (liquid crystal polymer) that has excellent heat resistance. When high heat resistance is required, it is desirable to use inorganic fiber paper or heat-resistant nonwoven fabric made of sheet-like ceramic fibers as the buffer member 33. When high heat resistance is not required, rubber or sponge made of silicone resin or fluorine resin can be used as the buffer member 33.

[0071] The temperature detection element 31 is electrically connected to a control unit that controls the heat generation of the heater 22 via two lead wires 35. The temperature detection element 31 and the buffer member 33 are provided on the lower surface of the holder 32 in FIG. 4B.

[0072] In this embodiment, the holder 32 is a longitudinal member formed long in one direction (the left-right direction in Figs. 4A to 4C), and the temperature detection element 31 and the buffer member 33 are provided at the center of the holder 32 in the longitudinal direction. Moreover, the holder 32 according to this embodiment is formed narrower at a central portion than at either end in the longitudinal direction, and the temperature detection element 31 and the buffer member 33 are provided at the central portion formed narrower.

[0073] 4B, protrusions 32b are provided on the upper surface of the holder 32 for positioning the coil spring 40. These protrusions 32b are provided on each of both ends of the holder 32 in the longitudinal direction.

[0074] The insulating sheet 34 is attached so as to collectively encase the temperature detection element 31, the holder 32, and the buffer member 33. The insulating sheet 34 is made of a resin such as polyimide that has good insulating properties, heat resistance, abrasion resistance, and thermal conductivity.

[0075] (● Safety device installation status) Figures 4B, 4C, and 5A show a state in which the safety device 25 is attached to the retaining member 23, which is a mating member. Figure 4B is a side view showing the mounting structure of the safety device 25, Figure 4C is a plan view showing the mounting structure of the safety device 25, and Figures 5A(a), (b), and (c) are xx, yy, and zz cross-sectional views, respectively, in Figure 4C. Note that the mounting structures of the safety devices 25, 26, 27, and 55 are similar to each other, so the mounting structure of one first safety device 25 will be described.

[0076] As shown in Fig. 4C, the safety device 25 is accommodated in a frame-shaped or groove-shaped accommodation portion 23a provided in the holding member 23. At this time, as shown in Fig. 4B and Fig. 5A(c), the position of the safety device 25 with respect to the holding member 23 is restricted by inserting the convex engagement portion 23b provided in the holding member 23 into the concave engagement portion 32a provided in the safety device 25 (positioning mechanism). That is, the movement of the safety device 25 in a direction intersecting the axial direction of the convex engagement portion 23b is restricted by engaging the concave engagement portion 32a with the convex engagement portion 23b. Details of this positioning mechanism will be described later.

[0077] As shown in Figures 4B and 4C, when the safety device 25 is accommodated in the accommodation portion 23a, the end of the holder 32 opposite to the end where the concave engagement portion 32a is provided is engaged with the opposing side wall surface 23c of the accommodation portion 23a. This restricts the rotation of the holder 32 around the convex engagement portion 23b. In this manner, the movement and rotation of the holder 32 relative to the holding member 23 are restricted, and the safety device 25 is positioned.

[0078] The cross-sectional shape of each of the concave engagement portion 32a and the convex engagement portion 23b may be a circle, a triangle, a rectangle, or another polygon. When these cross-sectional shapes are polygonal, it is possible to restrict the rotation of the holding member 23 around the convex engagement portion 23b.

[0079] As shown in Fig. 4B, in the embodiment of the present invention, concave engagement portion 32a is provided on the end side (right side in Fig. 4B) of holder 32 in the direction in which lead wire 35 extends, which makes it easier for an operator to assemble safety device 25 by gripping exposed portions of lead wire 35. In other words, since concave engagement portion 32a is provided at a position close to the position where an operator grips lead wire 35, concave engagement portion 32a can be easily inserted in alignment with convex engagement portion 23b, making the assembly easier.

[0080] Depending on the shape of the component to which the safety device 25 is attached and the layout of the surrounding components, the concave engagement portion 32a may be provided on the end side where the lead wire 35 of the holding body 32 is not exposed (the left side in Figure 4B), contrary to the embodiment of the present invention.

[0081] 4B and 4C, the holding member 23 is provided with a through hole 23d in which the temperature detection element 31 and its peripheral parts are disposed near the protruding engagement portion 23b. The temperature detection element 31 and the like (including the buffer member 33 and the insulating sheet 34) are disposed in this through hole 23d, so that the temperature detection element 31 comes into contact with the heater 22 via the insulating sheet 34. Alternatively, a highly thermally conductive member (heat equalizing member) made of aluminum, graphite, or the like may be disposed between the temperature detection element 31 and the heater 22, so that the temperature detection element 31 comes into contact with the heater 22 via this highly thermally conductive member (and the insulating sheet 34).

[0082] (Step of through hole) Step portions 23k are formed at multiple locations on the inner circumferential edge of the through hole 23d on the heater 22 side. Here, as shown in Fig. 4B and Fig. 4C, a pair of step portions 23k are formed at two locations facing each other in the longitudinal direction of the holding member 23.

[0083] The pair of steps 23k extend a predetermined length toward the mating step 23k in the longitudinal direction of the holding member 23. The pair of steps 23k support both end portions of the insulating sheet 34 of the safety device 25. The temperature detection element 31 abuts against the rear surface of the heater 22 at the intermediate position between the pair of steps 23k.

[0084] 4B, the member indicated by the reference numeral 41 is the stay 24 as a support member that supports a pair of coil springs 40 as biasing members that bias the safety device 25. When the safety device 25 is biased toward the heater 22 and the holding member 23 by the pair of coil springs 40, both end portions of the insulating sheet 34 and the temperature detection element 31 come into contact with the rear surface of the heater 22 with a predetermined pressure.

[0085] One end of each coil spring 40 is positioned by the two protrusions 32b provided on the safety device 25. By inserting each protrusion 32b into the end of the coil spring 40 and positioning it, the coil spring 40 is prevented from shifting or buckling, and a stable contact pressure can be applied.

[0086] Furthermore, the presence of the buffer member 33 between the temperature detection element 31 and the holder 32 ensures that the temperature detection element 31 comes into contact with the heater 22. That is, even if the safety device 25, the holder 23, or the like has a dimensional tolerance in the up-down direction in Fig. 4B, the buffer member 33 elastically deforms (compresses) in accordance with the dimensional tolerance, thereby ensuring that the temperature detection element 31 comes into contact with the heater 22. Furthermore, in order to allow for elastic deformation (compressive deformation) of the buffer member 33, a gap S1 is provided between the holder 23 and the holder 32 of the safety device 25.

[0087] In addition, the buffer member 33 is made of a material having a lower thermal conductivity and rigidity than the holder 32, and has elasticity and heat insulation properties. Therefore, the buffer member 33 also functions as a heat insulation member that reduces the heat transferred from the heater 22 to the holder 32.

[0088] (● Safety device positioning mechanism) Next, the positioning mechanism of the safety device 25 will be further described. As shown in Fig. 4B, a concave engaging portion (engaged portion) 32a for engaging the holder 32 with the convex engaging portion 23b of the holder 23, which is the mating member, is provided on the surface of the holder 32 on the side (heater side) where the temperature detection element 31 and the buffer member 33 are provided. The concave engaging portion 32a is provided on one longitudinal end side of the holder 32 where the lead wires 35 are exposed.

[0089] The convex engaging portion 23b and the concave engaging portion 32a constitute a positioning mechanism that positions the safety device 25 at a predetermined position on the heater 22 or the holding member 23. The concave-convex engaging structure of this positioning mechanism may be reversed between the holding body 32 of the safety device 25 and the holding member 23. That is, as shown in Fig. 4D, the holding member 23 is formed with the concave engaging portion 23e as an engaging portion, and the holding body 32 of the safety device 25 is formed with the convex engaging portion 32c as an engaged portion that engages with the concave engaging portion 23e.

[0090] 4B and 4D, the positioning mechanism is not limited to the concave-convex engagement structure formed on the mutually opposing surfaces of the holding member 23 and the holding body 32. For example, as shown in FIG. 4C, a part of the positioning mechanism can also be configured by the opposing side wall surfaces 23c of the accommodation portion 23a of the holding member 23.

[0091] (● Safety device mounting structure) Fig. 5B is a cross-sectional view showing the mounting structure of the safety device 25 to the holding member 23. (a) is a cross-sectional view of the first embodiment, (b) is a cross-sectional view of the second embodiment, (c) is a cross-sectional view of the third embodiment, and (d) is a cross-sectional view of the fourth embodiment. Fig. 5B(c) is a view of the safety device 25 as seen from the rear surface side of the heater 22.

[0092] 5B(a)-(d) all have a small gap S2 formed between the center of the heater side where the heat-sensitive surface of the safety device 25 is located and the back surface of the heater 22. This gap S2 keeps the center of the heat-sensitive surface of the safety device 25 out of contact with the heater 22. To form this gap S2, steps 23k are formed at multiple locations around the gap S2.

[0093] The step portion 23k is integrally formed on the inner periphery of the through hole 23d of the holding member 23 on the heater 22 side. The step portion 23k can be formed facing each other at two positions in the longitudinal direction of the holding member 23, as shown in Figs. 5B(a), (b), and (d).

[0094] 5B(b) shows an embodiment in which a heat equalizing member 28 is disposed between the heater 22 and the holding member 23. Since the temperature of the heater 22 is transferred to the heat-sensitive surface of the safety device 25 via the heat equalizing member 28, the detection accuracy of the safety device 25 can be stabilized.

[0095] 5B(c) shows an embodiment in which the through hole 23d is circular and the step portions 23k are formed at three locations at equal intervals in the circumferential direction of the through hole 23d. By forming the step portions 23k at three locations at equal intervals in the circumferential direction, the mounting posture of the safety device 25 is stabilized and the gap between the safety device 25 and the rear surface of the heater 22 is made uniform without bias. Therefore, the heat transfer from the heater 22 to the heat-sensitive surface of the safety device 25 can be improved, and the responsiveness of the safety device 25 can be improved.

[0096] Of the heat-sensitive surface of the safety device 25, the ratio Sa / Sb of the area Sa not in contact with the step portion 23k to the area Sb in contact with the step portion 23k is preferably 5 or more (5≦Sa / Sb). If the ratio Sa / Sb is less than 5, the proportion of the area Sb in contact with the step portion 23k is large, so that the amount of heat escaping from the heater 22 to the safety device 25 increases, which may result in poor fixing.

[0097] Also, it is desirable that an area within 2 mm inward from the outer periphery of the heat-sensitive surface of the safety device 25 abuts against the step portion 23k. If an area more than 2 mm inward from the outer periphery of the heat-sensitive surface of the safety device 25 abuts against the step portion 23k, the amount of heat escaping from the heater 22 to the safety device 25 increases, which may result in poor fixing or a decrease in the detection accuracy of the temperature detection element 31.

[0098] Fig. 5B(d) shows a case where a fluid substance 29 is interposed in the gap S2 between the heater 22 and the safety device 25. That is, the gap S2 may be an air layer as in Fig. 5B(a)(b), or the gap S2 may be filled with the fluid substance 29 as in Fig. 5B(d). Either method can eliminate the variation in heat transfer due to the contact state between solids.

[0099] Generally, a lubricant is applied to the surface of the heater 22 to reduce friction with the fixing belt 20, but there is a possibility that this lubricant may find its way into the safety device 25. If the lubricant finds its way into the gap S2 in FIG. 5B(a)(b), the heat transfer state of the safety device 25 will change.

[0100] By filling the space between the heater 22 and the safety device 25 with the fluid material 29 from the beginning, it is possible to prevent the change in the heat transfer state, and in turn to suppress a decrease in the detection accuracy of the temperature detection element 31. The fluid material 29 includes semi-solid materials such as grease.

[0101] Figure 10 shows a conventional thermostat mounting structure. In Figure 10(a), the entire surface of safety device 25, including the heat-sensitive surface, is in direct contact with the rear surface of heater 22. In Figure 10(b), the entire surface of safety device 25, including the heat-sensitive surface, is in contact with the rear surface of heater 22 via thin-walled portion 23l of holding member 23.

[0102] 10(a), when the entire surface of the safety device 25, including the heat-sensitive surface, is in direct contact with the rear surface of the heater 22, the responsiveness of the safety device 25 improves, but the amount of heat escaping from the heater 22 to the safety device 25 increases. Also, variations inevitably occur in the solid-to-solid contact state between the heater 22 and the safety device 25, and variations in heat transfer caused by the variations in the contact state cause variations in the responsiveness of the safety device 25.

[0103] 10(b), in a structure in which the entire surface of the safety device 25, including the heat-sensitive surface, comes into contact with the heater 22 via the thin-walled portion 23l of the holding member 23, there is a problem that the resin does not spread evenly to the thin-walled portion 23l during injection molding of the holding member 23, in addition to the aforementioned variation in responsiveness (resin short circuit). For this reason, there is a limit to how thin the thin-walled portion 23l can be made, and the responsiveness of the thermostat is sacrificed.

[0104] As in the present embodiment shown in FIG. 5B, by configuring the heater 22 and the safety device 25 so that their heat-sensitive surfaces are not in contact with each other and an air layer exists between them, it is possible to improve the responsiveness of the safety device 25 while solving the problems of heat transfer variation and responsiveness variation caused by the solid-to-solid contact state from the heater 22 to the safety device 25.

[0105] As a means for providing an air layer, only the outer periphery of the heat-sensitive surface of the safety device 25 is held by a step 23k of the holding member 23 in Fig. 5B. The step 23k is short in length and has a small area, so it is easy to make it thin. Therefore, the thinned step 23k allows the safety device 25 to be closer to the heater 22, and the responsiveness of the safety device 25 can be improved.

[0106] (● Other positioning mechanisms for safety devices) The positioning mechanism of the safety device described above with reference to Figures 4A to 4D can also be configured with a plurality of types of engagement structures shown in Figures 6A to 6E. In Figure 6A, a convex engagement portion 32d is formed on one side in the width direction of the holding body 32. This convex engagement portion 32d engages with a concave engagement portion 23f of the holding member 23.

[0107] 6B shows that convex engagement portions 32e are formed on both sides in the width direction opposite the longitudinal direction of the holding body 32. These convex engagement portions 32e engage with concave engagement portions 23g of the holding member 23. The positioning mechanism of FIG. 6A or the positioning mechanism of FIG. 6B may be provided on both longitudinal sides of the holding body 32.

[0108] 6C shows a convex engagement portion 32f formed on one longitudinal end side of the holding body 32. This convex engagement portion 32e engages with the concave engagement portion 23h of the holding member 23.

[0109] 6D shows that concave engagement portions 32g are formed on both widthwise sides of one longitudinal end of the holding body 32. The concave engagement portions 32g engage with the convex engagement portions 23i of the holding member 23. The positioning mechanism may be provided on both longitudinal ends of the holding body 32.

[0110] 6E, as shown in (a) at one longitudinal end of the holder 32, both side surfaces in the short direction are engaged with the opposing side wall surfaces 23c of the accommodation section 23a, and as shown in (b) at the other longitudinal end of the holder 32, a convex engagement portion 32h is formed on the heater side of the holder 32. The convex engagement portion 32h is engaged with the concave engagement portion 23j of the holding member 23.

[0111] In this way, various configurations are possible for the positioning mechanism of the safety device 25. In any positioning mechanism, the engaging width in the thickness direction of the holding body 32 (the biasing direction of the coil spring 40) is characterized by being significantly shorter than in the past.

[0112] (●Type of stay) The above-mentioned stay 24 can be of several types as shown in Figures 7A to 7C. The stay 24 in Figure 7A is made by crimping two L-shaped angle members 24a and 24b and welding or screwing them together. By abutting only both ends of the short side of the holding member 23 against the stay 24, the heat transfer area is reduced and heat loss is suppressed.

[0113] 7B uses one L-shaped angle bar 24c, and only one short end of the holding member 23 abuts against the stay 24, thereby further reducing the heat transfer area and suppressing heat loss. A belt guide / spring holder 81 is fixed to the stay 24.

[0114] 7C uses one channel-shaped angle bar 24d, and only both ends of the short side of the holding member 23 are in contact with the stay 24 to reduce the heat transfer area and suppress heat loss. A belt guide / spring holder 82 is fixed to the stay 24.

[0115] As shown in Fig. 8(a), the temperature detection element 31 may be arranged coaxially in the longitudinal direction with the position of the above-mentioned protrusion 32b that positions the coil spring 40. This allows the biasing force of the coil spring 40 to be directly applied to the temperature detection element 31, and the pressure required for accurate temperature detection can be stably maintained. In addition, the convex engagement portion 23b and the concave engagement portion 32a in Figs. 7A to 7C may also be arranged to overlap in the longitudinal direction in Fig. 8(a) in accordance with the position of the protrusion 32b in Fig. 8.

[0116] 8(b), the temperature detection element 31 and the buffer member 33 are covered with an insulating sheet 34. The temperature detection element 31 is disposed on the buffer member 33. Here, the thickness of the safety device 25 is 4 mm, and the thickness of the buffer member 33 including the insulating sheet 34 in the natural state is 1 mm, and the width is 4 mm.

[0117] When the safety device 25 is not tilted and the nip pressure acts on the front surface of the safety device 25, the thickness of the cushioning material 33 is uniformly reduced to 0.4 mm in the width direction. Then, the uniform compressive force (6 kPa) of the cushioning material 33 acts on the temperature detection element 31, which enables the temperature detection element 31 to accurately detect the temperature of the heater 22.

[0118] (●Other types of fixing devices) Further, in addition to the fixing device described above, the present invention is also applicable to fixing devices as shown in Figures 9A to 9C. Each fixing device shown in Figures 9A to 9C will be briefly described below.

[0119] 9A, a pressure roller 90 is disposed on the opposite side of the pressure roller 21 with respect to the fixing belt 20, and the fixing belt 20 is sandwiched and heated by the pressure roller 90 and heater 22. Meanwhile, on the pressure roller 21 side, a nip forming member 91 is disposed on the inner periphery of the fixing belt 20. The nip forming member 91 is supported by a stay 24, and the fixing belt 20 is sandwiched between the nip forming member 91 and the pressure roller 21 to form a nip portion N.

[0120] 9B, the above-mentioned pressure roller 90 is omitted, and in order to ensure the circumferential contact length between the fixing belt 20 and the heater 22, the heater 22 is formed in an arc shape in accordance with the curvature of the fixing belt 20. The rest of the configuration is the same as that of the fixing device 9 shown in FIG. 9A.

[0121] 9C, a pressure belt 92 is provided in addition to the fixing belt 20, and a heating nip (first nip portion) N1 and a fixing nip (second nip portion) N2 are separately configured. That is, a nip forming member 91 and a stay 93 are disposed on the opposite side of the pressure roller 21 from the fixing belt 20 side, and the pressure belt 92 is rotatably disposed so as to include the nip forming member 91 and the stay 93.

[0122] Then, the sheet P is passed through the fixing nip N2 between the pressure belt 92 and the pressure roller 21, where heat and pressure are applied to fix the image. The rest of the configuration is the same as that of the fixing device 9 shown in FIG. 2A.

[0123] Although the configurations of various fixing devices to which the present invention can be applied have been described above, the heating device according to the present invention is not limited to being applied to fixing devices. For example, the heating device according to the present invention can also be applied to a drying device mounted on an inkjet image forming device to dry ink applied to paper. Furthermore, the heating device according to the present invention can also be applied to a coating device (laminator) that overlaps a film as a coating member on the surface of a sheet and heats and presses them together, in addition to a heating device that heats paper as a heating target member. [Explanation of symbols]

[0124] 1Y, 1M, 1C, 1Bk: Imaging unit 2: Photoconductor 3: Charging device 4: Developing device 5: Cleaning device 6: Exposure device 7: Paper feeder 8: Transfer device 9: Fixing unit 10: Paper ejection unit 11: Intermediate transfer belt 12: Primary transfer roller 13: Secondary transfer roller 14: Paper transport path 15: Timing roller 19: Heating device 20: Fixing belt 21: Pressure roller 21a: Iron core 21b: Elastic layer 21c: Release layer 22: Heater 23: Holding member 23a: Storage section 23b: Convex engaging part 23c: Side wall surface 23d: Through hole 23e~23h, 23j: Concave engagement part 23i, 23z: Convex engagement part 23k: Stepped part 24, 41, 93: Stays 24a-24d: Angle bars 25-27, 55: Safety device 25a, 26a, 27a, 55a: Temperature detection unit 31: Temperature detection element 32: Holder 32a, 32g, 32y, 32z: Concave engaging part 32b: Projection 32c to 32f, 32h: protruding engagement portion 33: cushioning member 34: Insulation sheet 35: Lead wire 36: Spacer 40: Pressing member 59: Heating block 60A: Central heating section 60B: End heating portion 61, 61A to 61D: Electrode portion 62: Power supply line 70: Connector 71: Housing 72: Contact terminal 72a: Contact portion 81, 82: Belt guide / spring holder 90: Pressure roller 91: Nip forming member 92: Pressure belt 100: Image forming device 103: Image forming apparatus body N: Nip portion P, P1, P2: Paper S1, S2: Gap W1: Small size paper passing area W2: Large size paper passing area [Prior art documents] [Patent documents]

[0125] [Patent Document 1] Patent No. 4546233 [Patent Document 2] Japanese Patent Application Publication No. 8-305191 [Patent Document 3] JP 2002-110313 A

Claims

1. A heating device having a heating element and a safety device having a heat-sensitive surface for detecting the temperature of the heating element, the safety device stopping the supply of power to the heating element when the temperature of the heat-sensitive surface reaches or exceeds a predetermined temperature, A heating device characterized in that it has a retaining member that holds the heating member from the back side, the retaining member has a through hole that continues to the back side of the heating member, the through hole accommodates the safety device, and steps that hold the outer periphery of the heat-sensitive surface are formed at three locations circumferentially around the inner circumference of the through hole, and the steps maintain a non-contact between the center of the heat-sensitive surface of the safety device and the heating member.

2. 2. The heating device according to claim 1, wherein an area of ​​said heat-sensitive surface within 2 mm inside from an outer periphery thereof is in contact with said step portion.

3. 3. The heating device according to claim 1, wherein a ratio Sa / Sb of an area Sa of the heat sensitive surface not in contact with the step to an area Sb of the heat sensitive surface in contact with the step is set to 5 or more.

4. 4. The heating device according to claim 1, further comprising a biasing member for biasing the safety device toward the heating member.

5. 5. The heating device according to claim 1, further comprising a heat equalizing member disposed between said heating member and said safety device.

6. 5. The heating device according to claim 1, further comprising a fluid material disposed between the heating member and the safety device.

7. 4. The heating device according to claim 1, wherein a gap between an outer periphery of the safety device and an inner surface of the through hole is set to less than 2 mm.

8. A fixing device comprising the heating device according to claim 1 .

9. An image forming apparatus comprising the fixing device according to claim 8.

Images