JP4546233B2 - Image heating device - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image heating apparatus suitable for use as a heating and fixing apparatus mounted on an image forming apparatus such as a copying machine or a printer, and more particularly to an image heating apparatus provided with a thermal element that suppresses runaway of a heating body.

  For example, an image heating and fixing device in an image forming apparatus such as a copying machine, a printer, or a fax machine, that is, a toner made of heat-softening resin or the like by an appropriate image forming process means such as electrophotography, electrostatic recording, or magnetic recording. Various types of apparatuses have been conventionally known and are in practical use as image heating and fixing apparatuses that heat and fix a toner image formed on a recording material as a permanently fixed image.

  Typical apparatuses include a heat roller type apparatus and a film heating type apparatus.

  A film heating type heat fixing apparatus is described in Patent Document 1, and is a system (on-demand) that has high heat transfer efficiency and quick start-up of the apparatus.

  FIG. 10 is a schematic cross-sectional view of the main part in an example of a film heating type heat fixing apparatus. Reference numeral 16 denotes a heater as a heating element, which is a long and thin plate-shaped low heat capacity heater having a longitudinal direction in the drawing as a longitudinal direction. This heater 16 has an elongated and thin plate-shaped heater substrate having an insulating property and good thermal conductivity, and a heating resistor portion (an energizing heating resistor) provided on the substrate as a basic component. The temperature is rapidly raised by energizing the part, and the temperature is quickly lowered by de-energizing.

  Reference numeral 17 denotes a heater holder as a heating body support member, which has rigidity and heat insulation. The heater 16 is fitted and fixedly supported in a heater fitting groove formed along the length of the member on the lower surface of the member. is there.

  As described above, the heat-resistant film (fixing film) 27 as a flexible member is slid by pressing with the elastic pressure roller 22 against the exposed surface (heater surface) of the heater 16 supported by the heater holder 17. Unfixed as a material to be heated between the film 27 and the pressure roller 22 in the pressure nip portion (fixing nip portion) N formed by the heater 16 and the pressure roller 22 with the film 27 interposed therebetween The recording material P on which an image (toner image) t is formed and supported is introduced, and the fixing nip portion N is nipped and conveyed together with the film 27, whereby the heat of the heater 16 is applied to the recording material P through the film 27. Thus, the unfixed image t on the recording material P is heated and fixed on the surface of the recording material P. The recording material P passing through the fixing nip N is separated from the surface of the film 27 and conveyed.

  At this time, the thermal element 19 that operates with the heat of the heater 16 when the heater 16 is abnormally heated to urgently cut off the power supply to the heater 16 is brought into contact with the surface of the heater 16 opposite to the fixing film 27 contact surface side. If the heater 16 is heated abnormally (heater runaway, excessive temperature rise beyond the permissible level) due to an uncontrollable state of energization to the heating resistor part, the heater 16 is heated by the abnormal temperature rise. The element 19 is actuated, and energization to the heating resistor portion is urgently cut off.

  As the heat sensitive element 19, a device such as a thermal fuse or a thermo switch that cuts off the current to the heating resistor section when a predetermined temperature or higher is sensed is used.

  Since such a thermal element has a relatively large heat capacity, if it is brought into direct contact with the heater, the heater temperature in this contact area will drop, and the area where the thermal element in the longitudinal direction of the heater is not in contact with the area. As a result, the heater temperature is different, and image heating unevenness is likely to occur. On the contrary, if the thermal element is arranged in a non-contact manner with respect to the heater, the response of the thermal element is lowered.

As means for solving these problems, means as described in Patent Documents 2 and 3 have been proposed. That is, a resin spacer having a small contact area with the heater is provided between the thermal element and the heater, and when the temperature of the heater is abnormally increased, the spacer softens and the thermal element contacts the heater.
JP-A-4-44075 JP-A-8-305191 JP 2002-110313 A

  However, it has been found that even if the spacer softens due to abnormal heating of the heater, the thermal element does not contact the heater firmly, and the response of the thermal element may be slightly delayed.

  For example, as shown in FIG. 9, when the heater is not abnormally heated, the thermal element 19 is held by the spacer 25 with a gap with respect to the heater ((a) of FIG. 9). When the spacer 25 is softened when the temperature of the heater is abnormally increased, the thermal element 19 is pushed down by the spring 26 and is in close contact with the heater surface ((b) of FIG. 9). The state of FIG. 9B is a normal state when the spacer is softened. However, the softened state of the spacer 25 at the time of abnormal temperature rise of the heater is not necessarily uniform, and the thermal element 19 may be pushed down diagonally or may be caught in the middle ((c) in FIG. 9). found. In such a case, since the thermal element 19 is not in close contact with the heater surface, the energization cutoff operation is delayed.

  Further, the heat sensitive surface of the heat sensitive element 19 is not necessarily smooth. For example, the thermosensitive surface of a thermoswitch that is often used as a heat-sensitive element 19 for safety measures is a flat surface as shown in FIG. 11A by design, but has a central recess due to manufacturing tolerances (FIG. 11). (B)) and a central convex ((c) of FIG. 11) are possible. In the heat sensitive element 19 having such a heat sensitive surface, even if the heat sensitive element 19 is pressed against the heater surface by the urging member when the temperature rises abnormally, the heat sensitive surface is not sufficiently adhered to the heater surface, so that the shut-off operation of the heat sensitive element 19 is delayed. Turned out to be. In particular, such a problem is likely to occur when the heat sensitive element is configured to receive heat from a heater on a flat heat sensitive surface such as a thermo switch.

  The present invention for solving the above problems includes a heating element having a heating resistor that generates heat when energized, a thermal element that operates with heat due to abnormal temperature rise of the heating element, and interrupts energization to the heating resistor. In the image heating apparatus, a resin spacer is provided between the heating element and the thermal element, and the spacer has an opening, a frame, and a leg extending inward from the frame. It is characterized by.

  According to the above-described image heating apparatus configuration, it is possible to provide an apparatus that is excellent in energization cut-off response at the time of abnormal temperature rise of the heating body while suppressing uneven heating of the image.

(1) Example of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of a color image forming apparatus equipped with the image heating apparatus of the present invention. The color image forming apparatus of this example is an apparatus that obtains a full color image by superimposing four color toner images of yellow, cyan, magenta, and black using an electrophotographic method, and has a process speed of 90 mm / sec. The number of printed sheets per minute is 16 US letter size paper. The time (FPOT) to the first print (First Page Out) is about 15 seconds.

  Y, C, M, and K are four process cartridges that respectively form yellow, cyan, magenta, and black color toner images, which are arranged in order from the bottom to the top. Each of the process cartridges Y, C, M, and K includes a photosensitive drum 1 as an image carrier, a charging roller 2 as a charging unit, a developing unit 3 for developing an electrostatic latent image, and a cleaning of the photosensitive drum. A so-called all-in-one cartridge in which the means 4 and the like are collected in one container is used.

  Yellow toner is used for the developing means 3 of the yellow process cartridge Y, cyan toner is used for the developing means 3 of the cyan process cartridge C, magenta toner is used for the developing means 3 of the magenta process cartridge M, and black process cartridge K is used. The developing means 3 is filled with black toner.

  An optical system 5 that forms an electrostatic latent image by exposing the photosensitive drum 1 is provided corresponding to the four color process cartridges Y, C, M, and K. As the optical system 5, a laser scanning exposure optical system is used.

  In each of the process cartridges Y, C, M, and K, the optical system 5 performs scanning exposure based on the image data on the photosensitive drum 1 that is uniformly charged by the charging unit 2. An electrostatic latent image corresponding to the scanning exposure image is formed on the surface. By setting the developing bias applied to the developing roller of the developing means 3 from a bias power source (not shown) to an appropriate value between the charged potential and the latent image (exposed rear) potential, the toner charged to a negative polarity Is selectively attached to the electrostatic latent image on the photosensitive drum 1 and developed.

  That is, a yellow toner image is formed on the photosensitive drum 1 of the yellow process cartridge Y, a cyan toner image is formed on the photosensitive drum 1 of the cyan process cartridge C, and a magenta toner is applied on the photosensitive drum 1 of the magenta process cartridge M. A black toner image is formed on the photosensitive drum 1 of the black process cartridge K, respectively.

  The monochromatic toner images developed and formed on the photosensitive drums 1 of the process cartridges Y, C, M, and K are on the intermediate transfer member 6 that rotates at a substantially constant speed in synchronization with the rotation of the photosensitive drums 1. In the predetermined alignment state, the toner images are sequentially superimposed and primarily transferred, whereby an unfixed full-color toner image is synthesized and formed on the intermediate transfer body 6.

  In the present embodiment, an endless intermediate transfer belt is used as the intermediate transfer member 6, and is suspended and stretched around three rollers: a driving roller 7, a secondary transfer roller facing roller 14, and a tension roller 8. Yes, driven by the drive roller 7.

  A primary transfer roller 9 is used as a primary transfer unit of the toner image from the photosensitive drum 1 of each process cartridge Y, C, M, and K to the intermediate transfer belt 6. By applying a primary transfer bias having a polarity opposite to that of toner from a bias power source (not shown) to the primary transfer roller 9, the intermediate transfer belt 6 is placed on the photosensitive drum 1 of each process cartridge Y, C, M, K. In contrast, the toner image is primarily transferred.

  After the primary transfer from the photosensitive drum 1 to the intermediate transfer belt 6 in each of the process cartridges Y, C, M, and K, the toner remaining as a transfer residue on the photosensitive drum 1 is removed by the cleaning unit 4. In the present embodiment, blade cleaning using a urethane blade is used as the cleaning means 4.

  The above process is performed in the process cartridges Y, C, M, and K for each color of yellow, cyan, magenta, and black in synchronization with the rotation of the intermediate transfer belt 6, and the primary transfer toner for each color is transferred onto the intermediate transfer belt 6. The images are formed one after another. It should be noted that the above process is performed only for the target color during image formation of only a single color (monochromatic mode).

  On the other hand, the recording material P set in the recording material cassette 10 serving as a recording material (recording material) supply unit is fed by the feeding roller 11 and is opposed to the secondary transfer roller by the registration roller 12 at a predetermined control timing. It is conveyed to a nip portion between the intermediate transfer belt 6 suspended around the roller 14 and the secondary transfer roller 13 as a secondary transfer means.

  The primary transfer toner image formed on the intermediate transfer belt 6 is collectively collected on the recording material P by a bias having a polarity opposite to that of a toner applied from a bias application unit (not shown) to a secondary transfer roller 13 as a secondary transfer unit. Transcribed.

  The secondary transfer residual toner remaining on the intermediate transfer belt 6 after the secondary transfer is removed by the intermediate transfer belt cleaning means 15. In this embodiment, the intermediate transfer member is cleaned with a urethane blade, like the cleaning unit 4 of the photosensitive drum 1.

  The toner image secondarily transferred onto the recording material P passes through the fixing device F as a fixing unit, and is fused and fixed onto the recording material P. Then, the toner image is sent to the discharge tray 32 through the discharge path 31. It becomes an output image (full color print or mono color print) of the image forming apparatus.

(2) Fixing device (image heating device) F
FIG. 2 is a schematic configuration model diagram of the fixing device F. The fixing device F of this example is a pressure roller driving type (tensionless type) image heating device using a flexible rotating body (fixing belt).

1) Overall Configuration of Device F Reference numeral 20 denotes a fixing belt (flexible rotating body), which is a cylindrical member in which an elastic layer is provided on a polyimide base layer. The fixing belt 20 will be described in detail later in section 3).

  Reference numeral 22 denotes a pressure roller. Reference numeral 17 denotes a substantially semicircular arc-shaped cross section, and a vertical heat-resistant heater holder (heating body holding member). Reference numeral 16 denotes a heater (heating body) as a heat source, which is disposed on the lower surface of the heater holder 17 along the length of the holder. It is. The fixing belt 20 is loosely fitted on the heater holder 17. In this embodiment, the heater 16 is a ceramic heater which will be described in detail in section 2) below.

  The heater holder 17 is a molded product of liquid crystal polymer resin having high heat resistance. The heater holder of this embodiment serves not only to hold the heater 16 but also to guide the fixing belt 20. In this example, DuPont Zenite 7755 (trade name) was used as the liquid crystal polymer. The maximum usable temperature (heat resistant temperature) of Zenite 7755 is about 270 ° C.

  The pressure roller 22 is formed by forming a silicone rubber layer having a thickness of about 3 mm on a stainless steel core by injection molding and coating a PFA resin tube having a thickness of about 40 μm thereon. The pressure roller 22 is disposed such that both ends of the core bar are rotatably supported by bearings between a side plate (not shown) and a front side plate of the apparatus frame 24. On the upper side of the pressure roller 22, the heating assembly including the heater 16, the heater holder 17, the fixing belt 20, and the like is disposed in parallel to the pressure roller 22 with the heater 16 facing downward, and both ends of the heater holder 17 are not attached. It is urged toward the pressure roller 22 by a force of 98 N (10 kgf) on one side and a total pressure of 196 N (20 kgf) by the illustrated pressure mechanism. As a result, a fixing nip portion N having a predetermined width necessary for heat fixing is formed. The pressurization mechanism has a pressure release mechanism, and is configured to release the pressurization and remove the recording material P at the time of jam processing or the like.

  Reference numeral 18 denotes a thermistor as temperature detecting means. The thermistor 18 is elastically brought into contact with the inner surface of the fixing belt 20 above the heater holder 17 and detects the temperature of the inner surface of the fixing belt 20.

  The thermistor 18 is attached to the tip of a stainless steel arm fixedly supported by the heater holder 17. Since the arm has elasticity, the thermistor element is always kept in contact with the inner surface of the fixing belt 20 even when the movement of the inner surface of the fixing belt 20 becomes unstable.

  The thermistor 18 is electrically connected to a control circuit unit (CPU) 21. The control circuit unit 21 controls the heater drive circuit unit 28 (FIG. 3) based on the output of the thermistor 18. The control circuit unit 21 of this embodiment controls energization to the heater 16 (more precisely, to a heating resistor b described later) so that the detected temperature of the thermistor 18 maintains a predetermined control temperature. The control temperature in this embodiment is 180 ° C.

  Reference numeral 23 denotes an entrance guide assembled to the apparatus frame 24, and 34 denotes a fixing paper discharge roller. The entrance guide 23 serves to guide the recording material so that the recording material P that has passed through the secondary transfer nip accurately enters the fixing nip portion N. The entrance guide 23 of the present embodiment is formed of polyphenylene sulfide (PPS) resin.

  Reference numeral 19 denotes a thermo switch as a heat sensitive element, and 25 denotes a resin spacer disposed between the heater 16 and the thermo switch 19. The spacer 25 is inserted into a hole provided in a part of the heater holder 17. The thermo switch 19 is urged toward the back surface of the heater 16 by a spring 26 as an urging member via a spacer 25. The operating temperature of the thermoswitch of this embodiment is about 200 ° C.

  Reference numeral 25 denotes a resin spacer disposed between the heater and the thermo switch. The softening point of the spacer is set to be lower than the heat resistant temperature of the heater holder and higher than the operating temperature of the thermoswitch, and the softening point of the spacer of this embodiment is set to about 250 ° C. The operating temperature of the thermoswitch used in this embodiment is about 200 ° C. as described above, and a thermoswitch having such an operating temperature has an advantage that it can be obtained at a low price. However, since the difference between the operating temperature of the thermo switch and the control temperature of the heater is small, in a configuration in which a spacer is not provided (the thermo switch is in direct contact with the heater) or in a configuration using a spacer whose softening point is lower than the operating temperature of the thermo switch, Even if the heater overshoots slightly during normal operation, the thermoswitch will operate. On the other hand, if a spacer with a softening point higher than the operating temperature of the thermoswitch is used as in this embodiment, the thermoswitch can be prevented from operating when the heater is operating normally, and the heater malfunctions. If the spacer softens due to a rise in temperature, the thermoswitch is activated immediately, increasing the reliability of the safety function.

  The thermo switch 19 is connected in series with a power supply line (not shown) to the heater 16 and is configured to cut off the energization of the heater 16 when a temperature above a predetermined temperature is detected.

  The pressure roller 22 is rotationally driven by the driving means M in a counterclockwise direction indicated by an arrow at a predetermined peripheral speed. When the pressure roller 22 rotates, the cylindrical fixing belt 20 is driven to rotate by frictional force. Grease is applied to the inner surface of the fixing belt 20 to ensure slidability between the heater 16 and the heater holder 17 and the inner surface of the fixing belt 20.

  The recording material P on which the toner image is formed is heated and fixed by being nipped and conveyed at the fixing nip N. The recording material P that has passed through the fixing nip N is naturally separated from the fixing belt by the curvature of the fixing belt 20 and is discharged by the fixing discharge roller 33.

2) Heater (heating body) 16
The heater 16 as a heat source has an elongated plate-like substrate (insulating flat plate-like substrate) , a heating resistor layer formed on the substrate, and a heat-resistant glass layer covering the heating resistor layer.

  FIG. 3 is a structural model diagram of an example of such a ceramic heater, where (a) is a partially cutaway surface model diagram, (b) is a back model diagram, and (c) is an enlarged cross-sectional model diagram.

  The structure of the heater 16 will be described in detail with reference to FIG. a is a substrate made of alumina, b is a heating resistor layer formed by screen-printing a conductive paste containing a silver palladium (Ag / Pd) alloy on the substrate, the thickness is about 10 μm, and the width is 1 to 5 mm. Degree. c and d are electrode portions formed by screen-printing silver paste on the substrate in the same manner as the heating resistor layer, and a power feeding connector 27 is connected to the electrode portions c and d. e and f are electric circuit portions that electrically connect the electrode portions and the heating resistor layers, and are also screen-printed on the substrate. g is a glass coat layer having a thickness of about 10 μm formed on the heating resistor layer b and the electric circuit portions e and f in order to ensure protection and insulation. This glass coat layer is in contact with the inner surface of the fixing belt.

  Power is supplied from the heater drive circuit section 28 to the first and second electrode sections c and d via the power supply connector 27, so that the heating resistor layer b generates heat and the heater 16 quickly rises in temperature. The heater drive circuit unit 28 is controlled by a control circuit unit (CPU) 21.

  When a print command is input to the main body of the image forming apparatus, energization to the heater is started and the pressure roller 22 and the fixing belt 20 start to rotate. Energization of the heater 16 is controlled by PID control, and the input power is controlled so that the inner surface temperature of the fixing belt 20, that is, the detected temperature of the main thermistor 18 becomes 180 ° C.

3) Fixing belt 20
The fixing belt 20 is a heat-resistant resin film. The material is polyimide, and a base layer having a thickness of about 50 μm is provided on the base layer. The material is silicone rubber, and an elastic layer having a thickness of about 250 μm is provided on the elastic layer. It has a resin tube layer made of PFA and having a thickness of about 30 μm.

  For the silicone rubber layer, it is desirable to use a material having a high thermal conductivity as much as possible and to reduce the heat capacity of the fixing belt 20 from the viewpoint of quick temperature rise. In this example, a material having a thermal conductivity of about 4.19 × 10 −3 J / sec · cm · K and a silicone rubber having a high thermal conductivity were used.

  On the other hand, it is desirable to make the rubber layer of the fixing belt 20 as thick as possible from the viewpoint of image quality such as OHT permeability and uneven glossiness of the image after fixing. According to the study by the present inventors, it has been found that a rubber thickness of 200 μm or more is necessary to obtain a satisfactory level of image quality. The silicone rubber layer in this example was 250 μm thick.

The heat capacity of the fixing belt 20 thus formed was measured and found to be 1.17 × 10 ⁻¹ J / cm ² · K (heat capacity per 1 cm ^{2 of the} fixing belt). Generally, when the heat capacity of the fixing belt 20 is 4.19 J / cm ² · K or more, the temperature rise becomes dull and the on-demand property is impaired. On the other hand, if an attempt is made to reduce it to 4.19 × 10 ⁻² J / cm ² K or less, the rubber layer of the fixing belt 20 must be made extremely thin, and the OHT permeability, the level of gloss unevenness of the image, etc. The thickness of the rubber layer necessary for maintaining the image quality cannot be secured. Therefore, the on-demand property, the heat capacity of the fixing belt 20 that satisfies both of image quality, it is desirable that in a range of ^{4.19 × 10 -2 J / cm 2} K or more 4.19J / cm ² K .

  Further, by providing a fluororesin layer (in this embodiment, a PFA tube) on the surface of the fixing belt 20, the surface releasability is improved. Even if the toner once adheres to the surface of the fixing belt 20, the recording material P is again formed. In addition, the offset toner can easily move, and the offset phenomenon can be prevented.

  Further, by forming the fluororesin layer on the surface of the fixing belt 20 by covering the elastic layer with a tube, it is possible to more easily form a uniform fluororesin layer. .

(3) Operation of thermo switch 19 at abnormal temperature rise 1) Thermo switch (thermal element) 19
FIG. 4 shows a schematic cross-sectional view of the thermo switch 19. The thermoswitch case 50 (the lower part is a heat-sensitive surface) includes a bimetal 51 and a pin 52 disposed on the bimetal, and the pin 52 supports a current-carrying plate 53. The energizing plate 53 is in contact with the energizing plate 54 at the contact point 55, and the heater 16 can be energized via the contact point 55 as shown in FIG. It is in a state. On the other hand, when the heater 16 is abnormally heated, as shown in FIG. 4B, the bimetal 51 warps in the opposite direction to the normal state and pushes the pin 52 up. As a result, the energization plate 53 is also pushed up, the contact 55 is opened, and the heater cannot be energized.

  As can be seen from the above, it is preferable to transfer the heat of the heater to the center of the thermoswitch case 50 where the bimetal 51 exists in order to quickly operate the thermoswitch 19 when the heater 16 is abnormally heated.

FIG. 5 shows a detailed sectional view around the thermoswitch 19 in the present embodiment. 6A, 6B, 6C, and 6D show examples of shapes when the spacer 25 is viewed from the bottom surface side (heater side). FIG. 12 is a perspective view of the spacer, and FIG. 13 is a perspective view of the heater holder with the spacer placed thereon. The thermo switch 19 is biased by the spring 26 in the direction of the heater 16 (on the heating body side) with a biasing force of 4.9 N (500 gf). A resin spacer 25 is disposed between the thermo switch 19 and the heater 16.

  The spacer 25 is provided with a “rib” portion 25 e that engages with the periphery 17 a of the hole portion of the heater holder 17. When the thermo switch 19 is biased by the spring 26, the collar portion 25e hits the periphery 17a of the hole portion of the heater holder 17 and is not pushed further into the heater side. As a result, a large pressure is not applied to the heater 16. Further, since the collar portion 25e is large enough to abut the periphery 17a of the hole portion of the heater holder 17, the spacer 25 does not fall off from the heater holding surface side of the heater holder 17. Since it is such a structure, it is easy to assemble the heater 16, the heater holder 17, the spacer 25, the thermo switch 19, the spring 26, etc., and assemblability improves. In this embodiment, the thickness of the thermoswitch support surface of the spacer 25 is set to 0.5 mm. As described above, the softening point of the spacer 25 is about 250 ° C., and is set so as to soften when the heater 16 is abnormally heated.

  As shown in FIGS. 5, 6, and 13, the spacer 25 includes an opening 25b, a frame 25c around the opening 25b, and a leg 25d extending inward from the frame 25c. The presence of the leg portion 25d makes it easy for the heat of the heater 16 to be transferred to the thermo switch 19 when the spacer 25 is softened.

  The area ratio between the opening 25b and the leg 25d on the thermoswitch support surface of the spacer 25 described above may be set as appropriate according to the use state of the fixing device, the operating temperature of the thermal element 19, and the like. In addition, when the thermo switch 19 is used as the heat sensitive element, the thermo switch 19 has the internal configuration as described above. Therefore, in order to ensure the reliable operation of the thermo switch 19, the opening is the heat sensitive surface of the thermo switch case 50. In other words, a structure in which the leg portion 25d is in the central region of the heat sensitive surface is desirable.

  On the other hand, it is desirable to make the opening 25b as large as possible in order to prevent the thermoswitch 19 from operating at normal times when the heater is not abnormally heated due to heat transfer of the leg 25d. Moreover, the structure which enlarges an opening part is preferable also in the meaning which suppresses the thermal radiation from the heater to a spacer when the heater is not abnormally heated up.

  Further, if a plurality of openings 25b are formed by providing the leg portions 25d, the posture of the thermo switch 19 can be stabilized during normal times and during abnormal temperature rise.

  FIGS. 6A, 6B, 6C, and 6D show examples of the shape of the thermoswitch support surface of the spacer 25, respectively. These are examples when the circular thermoswitch case 50 is accommodated, and the present invention is not limited to these shapes. In this embodiment, the spacer 25 having the shape (a) is used. In the spacers of the shapes (a), (b), and (c), the leg portions 25d intersect at the center of the circular frame portion 25c, and the leg portions exist in the central region. The spacer (d) has a single leg 25d but a leg in the central region. That is, any shape has a structure in which a leg is present in the center.

  In addition, the center part (leg part) of the spacer is in contact with the thermoswitch heat-sensitive surface when the spacer is not softened, but by setting the area of the opening larger than the area of the leg part, Therefore, it is possible to prevent the thermoswitch from operating when the heater is not heated abnormally.

  As a material for the spacer 25, in this embodiment, a polyethylene terephthalate resin containing glass fiber having a softening temperature of about 250 ° C. was used. As described above, since the spacer has a softening point higher than the operating temperature (200 ° C.) of the thermo switch, it can prevent the thermo switch from operating when the heater is operating normally. If the softens, the thermoswitch will operate immediately, increasing the reliability of the safety function. The softening point (about 250 ° C.) of the spacer is set lower than the heat resistant temperature (about 270 ° C.) of the heater holder. With this setting, even if an abnormal temperature rise of the heater occurs, the spacer softens before the heater holder and the thermo switch operates, so that the heater holder can be reused without damaging the heater holder.

2) Operation at Abnormal Temperature Increase Conventionally, when the heater 16 is abnormally heated for some reason, the spacer 25 is softened, and the distance between the thermo switch 19 and the heater 16 is reduced. In the present invention, as shown in FIG. 7A, the distance between the thermo switch 19 and the heater 16 is reduced by softening and melting the spacer 25 by the heat at the time of abnormal heating of the heater. At the same time, by melting the thermoswitch support surface of the spacer 25, the space between the thermoswitch 19 and the heater 16 is filled with the melted resin material 25a of the spacer 25, and the heat of the heater 16 is transmitted to the thermoswitch 19 through this resin material 25a. The thermo switch 19 is operated by heating. Therefore, as shown in FIG. 7B, even if the position of the thermo switch 19 is tilted when the spacer is softened due to abnormal heating of the heater, resin corresponding to the leg portion before softening is interposed between the thermo switch and the heater. Since it exists, the thermoswitch can be quickly activated by heat conduction through the softened resin. The operation of the thermo switch 19 cuts off the power to the heater 16 and prevents the fixing device F from being damaged due to the abnormal temperature rise of the heater 16.

  FIG. 8 shows a graph in which 1000 W of power is supplied to the heater 16 without power control and the temperature increase rate on the back surface of the heater 16 is measured. As can be seen from this graph, the temperature of the back surface of the heater 16 reaches 300 ° C. in 5.5 seconds. Zenite 7755 used as the material of the heater holder 17 can be used up to 270 ° C., but if it exceeds 300 ° C., deformation occurs due to the heat of the heater 16 and the pressure of the pressure member. Before the temperature reaches 300 ° C., it is necessary to operate the thermo switch 19.

  When the runaway test was performed with the power of 1000 W applied using the fixing device of this example, the thermoswitch 19 operated 3.7 seconds from the start of power application, and the energization of the heater 16 was cut off.

  In addition, when the fixing device of this example was mounted on an image forming apparatus and used for normal image output, a good image without toner fixing failure and gloss unevenness on the toner surface was obtained.

  In the present embodiment, the thermo switch 19 is used as the heat sensitive element, but other elements such as a temperature fuse and a thermistor may be used. In particular, in the case of a heat sensitive element having a flat heat sensitive surface like the thermo switch of the present embodiment, the attitude of the thermo switch tends to be inclined when the spacer is softened, and the effect is great when such a heat sensitive element is used.

  In this embodiment, the flexible rotating body 20 of the fixing device is a type for the color image forming apparatus in which an elastic layer is provided on the surface of the fixing sleeve, but there is no elastic layer on the surface of the fixing sleeve. The present invention may be applied to a fixing device for a monochrome image forming apparatus. Even in a monochrome image type apparatus, the present invention is effective particularly in a configuration in which the back surface of the heater 16 becomes high due to high speed or the like.

  Further, in this embodiment, as the heating method of the fixing sleeve 20, the heater holder 17 and the heater 16 are arranged inside the fixing sleeve 20 and the heating method is adopted from the inner surface of the fixing sleeve. It is also possible to employ a fixing method in which the surface of the fixing sleeve 20 is directly contacted and heated. The present invention can also be applied to a fixing device using a rigid fixing roller. In this case, the fixing roller corresponds to a heating body, and the heat source disposed inside the fixing roller corresponds to a heating resistor. The spacer of this embodiment may be disposed between the surface of the fixing roller and the thermal element.

  As the material of the heater holder 17 of this embodiment, Zenite 7755 (trade name), which is a liquid crystal polymer (LCP), and polyethylene terephthalate as the material of the spacer 25 are used. Metal, other grades of LCP, and spacers 25 are made of various materials such as polyimide, polyphenylene sulfide (PPS), liquid crystal polymer (LCP), and heat resistant grade materials different from Zenite 7755. be able to. This combination method is free as long as the melting point of the heater holder 17 is set higher than the melting point of the spacer 25.

<Others>
1) The image heating apparatus of the present invention is not only an image heating and fixing apparatus, but also, for example, an image heating apparatus that heats a recording material carrying an image once fixed to improve the surface properties such as gloss, The present invention can be widely used as an image heating apparatus for processing, an image heating apparatus for supplying a sheet-like material for drying and laminating, an image heating apparatus for drying used in an inkjet printer, and the like.

  2) Although the ceramic heater having the structure illustrated in FIG. 3 is used as the heating body 16 in the embodiment, it is needless to say that a ceramic heater having a different structure may be used. A so-called back surface heating type ceramic heater in which the heating resistor b is provided on the surface of the substrate a opposite to the sliding surface of the flexible member may be used. Further, a heating body using nichrome wire or the like may be used as the heating resistor.

Schematic configuration model diagram of an example of an image forming apparatus in Embodiment 1 Sectional view of the fixing device in Embodiment 1 The figure which shows the structure of the heater (heating body) in Example 1. (A) is a figure which shows the normal time of a thermoswitch, (b) is a figure which shows the time of the heater abnormal temperature rise of a thermoswitch Detailed view of the periphery of the thermoswitch in Example 1 (A), (b), (c) is a figure which shows the form example of a spacer, respectively. (A) is a diagram showing when the heater is abnormally heated, and (b) is a diagram showing a case where the thermoswitch is in contact with the diagonal when the heated body is abnormally heated. The graph which shows the temperature increase curve of the heater 16 at the time of 1000W electric power input in Example 1 Explanatory drawing of the comparative example 1 Sectional view of a conventional fixing device (A), (b), (c) is a diagram showing the cross-sectional shape of the thermoswitch case Perspective view of spacer Perspective view of spacer mounted on heater holder

Explanation of symbols

16 Heater 17 Heater holder 19 Thermo switch 20 Fixing sleeve 25 Spacer 26 Energizing member

Claims (9)

In an image heating apparatus comprising: a heating element having a heating resistor that generates heat by energization; and a thermal element that operates with heat due to abnormal temperature rise of the heating element and interrupts energization to the heating resistor.
A resin spacer is provided between the heating element and the thermal element, and the spacer has an opening, a frame, and a leg extending inward from the frame.   The image according to claim 1, wherein the apparatus further includes a resin-made heating body holding member that holds the heating body, and a softening point of the spacer is lower than a heat resistant temperature of the heating body holding member. Heating device.   The image heating apparatus according to claim 2, wherein a softening point of the spacer is higher than an operating temperature of the thermal element.   The image heating apparatus according to claim 2, wherein the spacer is held by the heating body holding member.   And a biasing member that biases the thermal element toward the heating body, and the biasing member moves the thermal element toward the heating body as the spacer softens when the heating body is abnormally heated. The image heating apparatus according to claim 1, wherein the image heating apparatus has an urging force to be applied.   The image heating apparatus according to any one of claims 1 to 5, wherein the heating body includes an insulating flat substrate and the heating resistor provided on the substrate.   The apparatus further includes a flexible rotating body that moves while being in contact with the heating body, and a pressure roller that forms a nip portion with the heating body via the flexible rotating body. The image heating apparatus according to claim 1, wherein the image heating apparatus is characterized.   The image heating apparatus according to claim 7, wherein the flexible rotating body is a heat-resistant resin film. The rotating body of flexible, An apparatus according to claim 7 or claim 8, characterized in that an elastic layer.

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