JP2012078678A - Image heating device - Google Patents

Abstract

In an image heating apparatus in which a resistance heating element protrudes outside a nip portion, excessive heat generation outside the nip portion can be easily suppressed during normal use, and further, a nip portion that becomes relatively high when an abnormal temperature rise occurs. Suppressing excessive heat generation on the inner side with a thermal element to suppress cracking of the substrate of the heating element. [Solution] In an image heating apparatus in which a resistance heating element protrudes outside the nip, the temperature of the protrusion increases. Accordingly, the heat generation amount is suppressed, and a heat-sensitive element that interrupts energization when the temperature rises abnormally is disposed above the resistance heating element located inside the nip portion.
[Selection] Figure 1

Description

  The present invention relates to an image heating apparatus used in an image forming apparatus employing an image forming process such as an electrophotographic system or an electrostatic recording system, such as a copying machine or an LBP.

  As an image heating device, a fixing device that heats and fixes an unfixed toner image formed on a recording material as a fixed image, or a glossiness increasing device that increases the glossiness of an image by heating the image fixed on the recording material Etc.

  2. Description of the Related Art Conventionally, for example, in an image heating apparatus for a recording material for heat fixing of an image, a recording material as a material to be heated is provided by a heating roller maintained at a predetermined temperature and a pressure roller pressed against the heating roller. A heat roller system that heats while nipping and conveying is frequently used.

  Recently, instead of the heat roller method, there is a method including a heating body, a support body of the heating body (hereinafter referred to as a stay), and a flexible member that is conveyed while being pressed against the heating body. That is, a recording material is provided by having a pressure roller for closely attaching a recording material as a material to be heated to a heating body via a flexible member, and applying heat of the heating body to the recording material via a flexible member. This is a flexible member heating method in which an unfixed image formed and supported on a surface is heated and fixed on a recording material surface.

  As a heating body of this image heating apparatus, a structure in which a resistance heating element is formed on a ceramic substrate, the resistance heating element is heated by power feeding, and the recording material is heated is general. The temperature of the heating body is controlled by a temperature control system. That is, the temperature of the heating body is detected by a temperature measuring element such as a thermistor that is in contact with or bonded to the heating body, and the temperature of the heating body is controlled by the CPU based on the detected temperature. In such a flexible member heating type image heating and heating apparatus, a heating body having a low heat capacity can be used as the heating body. For this reason, it is possible to save power and shorten the wait time (quick start) as compared with a conventional heat roller type apparatus which is a contact heating type.

  Here, in the image heating apparatus of the flexible member heating system, when the recording material is heated, moisture in the recording material becomes water vapor, and unfixed toner on the recording material is blown off when ejected from the printing surface of the recording material. The image may be disturbed. That is, a phenomenon unique to the flexible member heating method, such as “tailing”, may occur. Trailing is likely to occur when the amount of moisture in the recording material is large or when the force for electrically attracting toner to the pressure roller inside the nip portion of the image heating apparatus is weak.

  In order to prevent such tailing, a part of the resistance heating element protrudes to the upstream side of the nip of the image heating apparatus, and the unfixed image on the recording material before entering the nip is preliminarily formed by the heat of the protruding resistance heating element. The structure which heats and heat-supposes is assumed (patent document 1, patent document 2).

  At this time, if the resistance heating element protrudes to the outside of the nip portion of the image heating apparatus, the protruding portion does not lose heat to the recording material or the pressure roller, so that the temperature rapidly increases. Then, only a part (upstream side) of the ceramic substrate expands, and the ceramic substrate is subjected to different thermal stresses in the vicinity of the upstream resistance heating element and in the vicinity of the downstream heating element, and the ceramic substrate may break during normal use. There is.

  In order to suppress this in advance, it is necessary to consider cracks in the ceramic substrate by suppressing the amount of heat generated as the temperature rises at the portion protruding outside the nip portion.

  Here, it is generally known to provide a thermal element (thermo switch, thermal fuse, etc.) that cuts off the power supply to the resistance heating element when an abnormal temperature rise occurs. When the thermal element is activated, if the image heating device itself has not been damaged, the image heating apparatus can be used again by simply cooling the thermal element and returning it to the conductive state or simply replacing the thermal element. Become.

Japanese Patent Laid-Open No. 10-186911 Japanese Patent Laid-Open No. 6-314041

  The thermal element that cuts off the power supply to the resistance heating element when there is an abnormal temperature rise is located at the position outside the nip (in the center of the heating element or on the resistance heating element side upstream) in the image heating device described above. It is thought that it is done.

  However, when a thermal element is provided in the center of the heating element or on the upstream side of the resistance heating element, it is necessary to adopt a configuration in which the amount of heat generation is suppressed as the temperature rises at the portion that protrudes outside the nip during normal use. The temperature increase rate of the part of the heating body is slow. For this reason, it has been found that it is difficult to cut off the energization of the resistance heating element with a sufficient margin when the temperature rises abnormally.

  That is, when the temperature rises abnormally, a large current flows through the resistance heating element, so that the temperature rapidly rises before the heat of the resistance heating element is transmitted to the ceramic substrate and surrounding components. In such a situation, whether or not the resistance heating element protrudes outside the nip does not significantly affect the temperature rise rate of the resistance heating element, and the resistance heating element on the downstream side depends on the temperature characteristics of the resistance heating element. The body temperature will rise rapidly compared to the upstream side.

  In this case, contrary to normal use, there is a possibility that the ceramic substrate cracks due to the thermal stress due to the temperature rise of the downstream resistance heating element.

  If the ceramic substrate is cracked, it is necessary to replace an expensive heater when repairing the image heating apparatus, which increases repair costs. Furthermore, when the ceramic substrate cracks, other parts such as a heater holding member may be damaged, and the image heating apparatus itself may need to be replaced.

  An object of the present invention is to easily suppress excessive heat generation outside the nip portion during normal use in an image heating apparatus in which the resistance heating element protrudes outside the nip portion. Furthermore, it is in suppressing the board | substrate crack of a heating body by restraining excessive heat_generation | fever inside the nip part which becomes comparatively high at the time of abnormal temperature rise with a thermal element.

  In order to achieve the above object, a typical structure of an image heating apparatus according to the present invention is a flexible structure in which a heating body and a recording material carrying one surface in contact with the heating body and the other surface in contact with an image are supported. A nip portion is formed between the pressure member that has a member and is pressed against the flexible member, and the flexible member and the recording material move with respect to the heating member, whereby the heating member heats. In the image heating apparatus, the heating element includes two or more resistance heating elements on a substrate, and at least one of the resistance heating elements is the nip portion. At least one of the resistance heating elements or a part thereof is positioned outside the nip portion to preheat the recording material upstream of the nip portion, and the nip portion The resistance heating element located inside and the outside of the nip portion The resistance heating element is electrically connected in series or in parallel, and the resistance heating element located inside the nip portion and the resistance heating element located outside the nip portion are connected in series. The resistance heating element positioned outside the nip portion has a negative resistance temperature characteristic, the resistance heating element positioned inside the nip portion, and the resistance heating element positioned outside the nip portion; Are connected in parallel, the resistance heating element located outside the nip portion has a positive resistance temperature characteristic, and the temperature control system controls the temperature of the resistance heating element located inside the nip portion. And a thermal element arranged close to or in contact with the heating body at a position corresponding to the inside of the nip, and shuts off power supply to the heating body when the heating body is abnormally heated. And

  According to the present invention, it is possible to apply preheating (preheating) to the recording material in order to reduce the tailing phenomenon during normal use, and excessive heat generation outside the nip portion where there is a possibility of substrate cracking. It can be easily suppressed. Furthermore, it is possible to reliably suppress excessive heat generation inside the nip portion, which becomes relatively high with respect to the outside of the nip portion when the temperature rises abnormally, by a heat sensitive element at a predetermined position, thereby suppressing the substrate cracking of the heating body. Become.

1 is a schematic configuration diagram of an image heating apparatus according to a first embodiment of the present invention. It is a schematic block diagram which shows the principal part of the laser beam printer which mounts the image heating apparatus which concerns on 1st Embodiment. It is a plane model figure of the heating body concerning a 1st embodiment. It is the schematic of the resistance temperature characteristic of the resistance heating element which concerns on 1st Embodiment. It is a conceptual diagram of the temperature distribution of the heating body width direction at the time of normal use which concerns on 1st Embodiment. It is a conceptual diagram of the temperature distribution of the heating body width direction at the time of normal use and abnormal temperature rise according to the first embodiment. (A) is a schematic block diagram of the 1st comparative example by which the thermal element position was arrange | positioned on the upstream heat generating body of a heating body, (B) is the 2nd by which this thermal element position was arrange | positioned in the center part of a heating body. It is a schematic block diagram of a comparative example. It is a conceptual diagram which shows the difference of the temperature rise curve at the time of abnormal temperature rise by the position of a thermal element, and the rise curve of a thermal stress. It is a plane model figure of the heating body which concerns on 2nd Embodiment. It is the schematic of the resistance temperature characteristic of the resistance heating element based on 2nd Embodiment. It is a plane model figure of the heating body which concerns on 3rd Embodiment. It is the schematic of the resistance temperature characteristic of the resistance heating element which concerns on 3rd Embodiment. It is a conceptual diagram of the temperature distribution of the heating body width direction at the time of normal use and an abnormal temperature increase according to the third embodiment. It is a plane model figure of the heating body which concerns on 4th Embodiment. It is the schematic of the resistance temperature characteristic of the resistance heating element which concerns on 4th Embodiment. It is a conceptual diagram of the temperature distribution of the heating body width direction at the time of normal use and an abnormal temperature increase according to the fourth embodiment. (A) is explanatory drawing at the time of normal use of a thermal element, (B) is explanatory drawing at the time of abnormal temperature rise of a thermal element. It is a figure in case a resistance heating element is comprised by three. (A) is a diagram when the resistance heating element outside the nip is provided on the downstream side of the nip, and (B) is a diagram when the resistance heating element provided on the downstream side of the nip is partially protruding from the nip. It is. It is a figure in case the upstream and downstream resistance heating elements are provided above the substrate.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings of the following embodiments, the same or corresponding parts are denoted by the same reference numerals.

<< First Embodiment >>
(Image forming device)
FIG. 2 shows a schematic configuration diagram of an example of an image forming apparatus equipped with the image heating apparatus according to the embodiment of the present invention. This image forming apparatus is a laser beam printer using an electrophotographic system. In FIG. 2, 101 is an organic photosensitive drum as an image carrier, 102 is a charging roller as a charging member, and 103 is a laser exposure apparatus. Reference numeral 104 denotes a developing device composed of a developing sleeve and a developing blade and one-component magnetic toner, 105 denotes a cleaning blade, 106 denotes a transfer roller, and 107 denotes an image heating device that performs heat fixing. The drum 101 is uniformly charged with negative charges by the charging roller 102, and an electrostatic latent image is formed on the drum 101 by the laser beam from the exposure device 103.

  Next, the toner charged in the developing device 104 adheres to the electrostatic latent image on the drum 101 to become a visible image, is transferred onto the recording material paper on the transfer roller 106, and is fixed by the image heating device 107. Is done. The cleaning blade 105 removes toner remaining on the drum 101. An image is formed by the function of each unit described above.

(Image heating device)
FIG. 1 is a schematic configuration diagram of an example of an image heating apparatus (flexible member heating method) in the present embodiment. In this image heating apparatus, an endless belt or cylindrical member is used as the flexible member 2, and at least a part of the circumference of the flexible member 2 is always tension-free (a state in which no tension is applied). The flexible member 2 is externally fitted to the stay 1 which is a guide member of the flexible member so as to include the heating body 3 so as to be rotationally driven by the rotational driving force of the pressure roller 4 as a pressure body. .

  That is, the flexible member 2 slides in contact with the heating member 3 on one surface and contacts the recording material P carrying the image on the other surface, and the flexible member 2 and the recording material P move relative to the heating member 3. Thus, the heat of the heating body 3 is transmitted to the recording material P through the flexible member 2.

  The inner peripheral length of the endless flexible member 2 and the outer peripheral length of the stay 1 including the heating body 3 are, for example, about 3 mm larger than that of the flexible member 2. It is fitted with a margin in length. In the present embodiment, the outer peripheral length of the flexible member 2 is 56.54 mm (in the cylindrical state, the diameter is 18 mm (φ18)).

  The pressure roller 4 forms a nip portion N with the flexible member 2 sandwiched between the pressure roller 4 and the pressure roller 4 as a flexible member outer surface contact driving means for rotationally driving the flexible member 2. It is a member. A rotational force acts on the flexible member 2 by the frictional force between the pressure roller 4 and the outer surface of the flexible member 2 and the rotational driving of the pressure roller 4. The pressure roller 4 also serving as a driving roller for the flexible member 2 includes a cored bar 4a, an elastic body layer 4b, and an outermost release layer 4c. And it arrange | positions so that the flexible member 2 may be pinched | interposed with the predetermined pressing force by the bearing means and the urging | biasing means not shown, and the surface of the heating body 3 may be pressed.

  In this embodiment, the cored bar 4a is an iron cored bar, the elastic body layer 4b is coated with silicone rubber, and the release layer 4c is coated with PFA. The outer diameter of the pressure roller 4 was 20 mm, and the thickness of the elastic layer 4b was 3 mm.

  The stay 1 is composed of a high heat resistant resin such as polyimide, polyamideimide, PEEK, PPS, or liquid crystal polymer, or a composite material of these resins with ceramics, metal, glass, or the like. In this embodiment, a liquid crystal polymer is used.

  In order to reduce the heat capacity and improve the quick start property, the flexible member 2 can use a single layer flexible member such as PTFE, PFA, FEP having heat resistance of 100 μm or less. . Alternatively, a composite layer flexible member in which PTFE, PFA, FEP or the like is coated on the outer peripheral surface of a flexible member such as polyimide, polyamideimide, PEEK, PES, or PPS can be used. In addition, a composite layer flexible member in which PTFE, PFA, FEP or the like is coated on the outer peripheral surface of a stainless steel flexible member having a film thickness of 40 μm or less can also be used.

  In this embodiment, a polyimide flexible member having a film thickness of about 50 μm coated with a release layer obtained by blending PTFE / PFA on the outer peripheral surface is used.

(Configuration of heating element during normal use)
The heating body 3 is an elongated shape having a longitudinal direction perpendicular to the conveyance direction “a” of the flexible member 2 and the recording material P, and has a heat resistance, insulation, and good thermal conductivity, and a substrate 31. Resistance heating elements 6 and 7 formed along the longitudinal direction of the front surface side. The two resistance heating elements, that is, the upstream resistance heating element 6 disposed upstream of the nip portion and the downstream resistance heating element 7 positioned inside the nip portion are folded back into two. Are electrically connected in series.

  The heating element 3 is exposed on the lower surface side of the stay 1 by exposing the surface (surface facing the nip portion which is a sliding surface of the flexible member) of the substrate 31 provided with the resistance heating elements 6 and 7 downward. It is held and fixedly arranged. Furthermore, a heat-resistant overcoat layer 34 (FIG. 3) that protects the surface of the heating element on which the resistance heating element is formed, and power supply electrodes 21 and 22 (FIG. 1) of the resistance heating element 32 are provided. It is a heating element.

  The triac (bidirectional thyristor) 25 connected to the AC power supply 26 in FIGS. 1 and 3 can flow current in both directions by connecting two complementary thyristors in antiparallel. It can be used not only for direct current but also for alternating current. The thyristor is a three-terminal device that can conduct between the anode and the cathode mainly by flowing the gate from the gate to the cathode.

  In FIG. 1, a temperature measuring element 5 is provided at a position corresponding to the inside of the nip portion on the back surface (non-flexible member sliding surface) of the heating body 3, and the temperature control system configured with the CPU 24 causes the nip portion. The temperature of the downstream resistance heating element 7 located inside is controlled within a predetermined range.

  That is, the temperature of the heating element 3 is increased when the resistance heating element generates heat over the entire length by feeding power to the feeding electrodes 21 and 22 at the longitudinal ends of the resistance heating element. The temperature rise is detected by the temperature sensing element 5, and the output of the temperature sensing element 5 is A / D converted and taken into the CPU 24. Based on the information, the electric power supplied to the resistance heating element by the triac 25 is controlled by phase, wave number control, etc. Thus, the temperature of the heating element 3 is controlled. In other words, the heating body 3 is fixed at a fixed temperature by controlling energization so that the heating body 3 is heated when the temperature detected by the temperature detecting element 5 is lower than a predetermined set temperature, and the temperature is lowered when the temperature is higher than the set temperature. To be kept.

  The substrate 31 of the heating body 3 is made of a material such as alumina or aluminum nitride as ceramics. In this embodiment, an alumina substrate having a thickness of 1 mm, a width of 6.4 mm, and a length of 270 mm is used. The power feeding electrodes 21 and 22 were formed using a screen printing pattern of silver / palladium alloy. The overcoat layer 34 was made of heat-resistant and slidable glass having a thickness of about 50 μm, and was formed by screen printing and fired in the same manner as the electrode pattern for power supply, thereby obtaining a heating element having desired characteristics.

  The resistance heating element is formed, for example, by applying an electric resistance material having negative resistance temperature characteristics such as ruthenium oxide, carbon, silicon carbide or the like in a linear or belt shape by screen printing or the like and baking it. is there. In this embodiment, a mixture of ruthenium oxide and glass and a dispersion of a temperature characteristic adjusting agent such as MgO or NbO are screen-printed separately, upstream and downstream, respectively, with a thickness of about 10 μm, a width of 1.0 mm, and a total length of 220 mm. Then, it was fired and formed.

  Regarding the position of the resistance heating element in the substrate width direction, the distance from the center of the resistance heating element 6 on the upstream side to the center of the substrate and the distance from the center of the resistance heating element 7 on the downstream side to the center of the substrate are equal. did. Specifically, the distance from the center of each resistance heating element to the center of the substrate was 1.9 mm. The distance from the both ends of the substrate in the substrate width direction to the resistance heating elements 6 and 7 was 0.7 mm.

  In a state where the temperature of the heating body 3 rises to a predetermined temperature and the rotational peripheral speed of the flexible member 2 is stabilized by the rotation of the pressure roller 4, the heating body 3 is pressed with the flexible member 2 interposed therebetween. The recording material P is introduced from the image forming portion (transfer portion) into the nip portion N formed by the roller 4. Then, when the recording material P is nipped and conveyed together with the flexible member 2, the heat of the heating body 3 is applied to the recording material P via the flexible member 2, and the recording material P The unfixed visible image (toner image) T is heat-fixed on the recording material P surface. The recording material P that has passed through the pressure nip N is separated from the surface of the flexible member 2 and conveyed.

  As shown in FIG. 1, in the present embodiment, the heating body has a structure protruding 2 mm upstream of the nip portion, and the upstream side of the resistance heating element 6 on the upstream side of the heating body is outside the nip portion. Is located.

  In the present embodiment, the entire resistance heating element 6 on the upstream side is located on the upstream side of the nip portion, but a part thereof is inside the nip portion, and the other portion protrudes outside the nip portion. Even in such an arrangement, the same effect can be expected at the outer portion of the nip portion. In FIG. 1, the resistance heating element 7 on the downstream side is located inside the nip portion.

  In the present embodiment, the temperature detecting element 5 is positioned inside the nip portion, and detects a temperature change inside the nip portion when the paper is passed. Therefore, the current value of the current I flowing through the upstream and downstream resistance heating elements is an amount that satisfies the heat generation amount W2 necessary for the temperature adjustment inside the nip portion, and as a result, good fixability can be obtained.

(Configuration of thermal element)
17A and 17B are schematic cross-sectional views as specific examples of the thermal element 11. 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 through the contact point 55 as shown in FIG. It has become. On the other hand, as shown in FIG. 17B, when the heater 16 is abnormally heated, the bimetal 51 warps in the opposite direction to the normal state and pushes up the pin 52. As a result, the energization plate 53 is also pushed up, the contact 55 is opened, and the heater cannot be energized.

(Suppression of heat generation due to temperature rise of upstream resistance heating element)
In this embodiment, by changing the amount of the temperature characteristic adjusting agent, the resistance change rate of the upstream resistance heating element 6 is −5000 ppm / ° C., and the resistance change rate of the resistance heating element inside (downstream) the nip portion. Was -300 ppm / ° C.

  The positions in the substrate width direction are arranged so that the distance from the center of the upstream resistance heating element 6 to the center of the substrate is equal to the distance from the center of the downstream resistance heating element 7 to the center of the substrate (each resistance heating element). The distance from the center to the substrate center was 1.9 mm).

  FIG. 4 shows a schematic diagram of resistance temperature characteristics of the upstream resistance heating element 6 and the downstream resistance heating element 7 in the present embodiment. In FIG. 4, the solid line indicates the resistance temperature characteristic of the upstream resistance heating element 6, and the broken line indicates the resistance temperature characteristic of the downstream resistance heating element 7.

Table 1 below shows the temperature change of the resistance value and the heat generation ratio of the upstream and downstream. Assuming that the resistance value of the upstream resistance heating element is R1 and the resistance value of the downstream resistance heating element is R2, the current I flowing through both resistance heating elements is common. The calorific values W2 of the side resistance heating elements are W1 = I ² × R1 and W2 = I ² × R2, respectively. Therefore, the heat generation ratio of the upstream and downstream resistance heating elements is the same as the resistance value ratio of the upstream and downstream resistance heating elements.

As in this embodiment, when the upstream resistance heating element R1 is located outside the nip portion, during normal use, R1 has a negative resistance temperature coefficient, so that R1 decreases as the temperature rises. As a result, W1 also decreases. Therefore, the temperature rise of the upstream heating element is suppressed, and the temperature difference in the ceramic substrate is reduced.

  That is, by connecting a plurality of resistance heating elements in series and the resistance heating element (upstream resistance heating element) located outside the nip portion has a negative temperature characteristic, upstream resistance heating is performed. When the body temperature rises, the resistance value of the upstream resistance heating element decreases. As a result, the voltage shared with the resistance heating element (downstream heating element) located inside the nip portion changes, and the power consumed by the downstream resistance heating element increases. Therefore, by suppressing the heat generation of the upstream resistance heating element, it is possible to suppress the problem that the upstream side of the ceramic substrate suddenly becomes high temperature and the problem that the ceramic substrate breaks during normal use.

(Status of heated body during abnormal temperature rise)
When the temperature rises abnormally beyond normal use, the resistance difference between the upstream resistance heating element R1 and the downstream heating element R2 becomes too large, and the temperature rise of the downstream resistance heating element R2 increases the temperature of the upstream resistance heating element R1. Significantly increases with the rise. Therefore, contrary to the normal use, the temperature of the downstream resistance heating element R2 is high, and the temperature of the upstream resistance heating element R1 is relatively low, resulting in a temperature difference between the resistance heating elements R1 and R2. The ceramic substrate is liable to crack due to the difference in thermal stress associated therewith.

  During normal use, when the downstream resistance heating element 7 inside the nip was temperature-controlled at 200 ° C., the total power of the heating element was about 500 W. At this time, the upstream resistance heating element 6 outside the nip portion consumed about 100 W of electric power, which was about 1/5 of the total electric power. In this state, the temperature of the resistance heating element 6 outside the nip portion (upstream side) and the resistance heating element 7 inside the nip portion (downstream side) are both saturated at about 200 ° C., as shown in FIG. Furthermore, the temperature distribution becomes symmetrical in the heating body width direction.

  However, when the power control becomes impossible for some reason, such as when the triac 25 is short-circuited, and the heating element 3 is abnormally heated, the resistance value of the upstream resistance heating element and the downstream resistance heating are as shown in Table 1. The ratio of body resistance is further increased. At the same time, since the overall resistance is lowered, a larger amount of electric power is input to the downstream resistance heating element. For example, when the temperature rises to 300 ° C., the electric power supplied to the heating body becomes 1010 W even when 100 V is input. As a result, as shown in FIG. 6, when a temperature difference opposite to that during normal use occurs in the substrate and the resulting thermal stress exceeds the minimum thermal stress at which the ceramic substrate cracks, Substrate cracking will occur.

(Control of heating element during abnormal temperature rise)
A thermal element 11 is provided that is close to or in contact with the back surface of the heating element 3 at a position corresponding to the inside of the nip portion, and that cuts off the power supply to the heating element 3 when the heating element 3 is abnormally heated (see FIG. ). As the thermosensitive element 11, a thermo switch, a thermal fuse, or the like can be used. In this embodiment, a thermo switch having a mechanism capable of interrupting current when the bimetal is inverted at a predetermined temperature is used. The operating temperature of the thermosensitive element 11 was 250 ° C. when measured in oil rising at a rate of 1 ° C./1 min.

  In the present embodiment, since the thermal element 11 is disposed above the downstream resistance heating element 7 of the heating element 3, an abnormal temperature rise of the downstream resistance heating element 7 is detected and before the ceramic substrate crack occurs. In addition, it is possible to cut off the power supply to the heating element 3.

7A and 7B, as a comparative example for the present embodiment, an image heating apparatus in which the thermal element 11 is arranged on the upstream heating element of the heating element, and the thermal element 11 are shown. The schematic sectional drawing of the image heating apparatus arrange | positioned in the center part of is shown.
FIG. 7 shows changes in the temperature of the thermal element at the time of abnormal temperature increase in the first embodiment and these comparative examples. As shown in FIG. 8, the temperature increase rate is faster in the case where the thermal element 11 is arranged on the downstream resistance heating element than in the case where the thermal element 11 is arranged on the central side and the upstream resistance heating element. Yes.

  Although there is a slight timing lag due to the heat capacity of the bimetal inside the thermal element and the operating time of the bimetal, the thermal element generally operates when the temperature of the thermal element reaches the operating temperature, and energization of the heating element is interrupted. The

  Further, FIG. 8 also shows a graph showing the transition of the thermal stress received by the ceramic substrate in the downstream resistance heating element portion at the time of abnormal temperature rise. When the thermal stress received by the ceramic substrate in the downstream resistance heating element portion at the time of abnormal temperature rise exceeds the value of the minimum thermal stress at which the ceramic substrate breaks, there is a high possibility that the ceramic substrate will crack.

  Here, when a thermal element was arranged in the middle part between the upstream resistance heating element and the downstream resistance heating element, a runaway test was performed in which an abnormal temperature increase was intentionally generated by a short circuit of the triac. Then, a large thermal stress was generated on the ceramic substrate in the downstream resistance heating element portion, which led to cracking of the ceramic substrate. From this, in the comparative example, it was found that the thermal stress received by the ceramic substrate of the downstream heating element exceeded the minimum thermal stress at which heater cracking occurred before the thermal element temperature reached the operating temperature. It was.

<< Second Embodiment >>
In the present embodiment shown in FIG. 9, the upstream resistance heating element 9 is a mixture of ruthenium oxide, glass, and a temperature characteristic adjusting agent, as in the first embodiment, and the downstream resistance heating element 10 is a silver / palladium alloy. A mixture of glass and glass was used. Then, the upstream resistance heating element 9 and the downstream resistance heating element 10 were formed on the alumina substrate by screen printing. In the present embodiment, the resistance change rate of the upstream resistance heating element 9 is −4000 ppm / ° C., and the resistance change rate of the downstream resistance heating element 10 is +500 ppm / ° C.

  FIG. 9 shows a front view of a heating body and a circuit for performing energization control in this embodiment. The length, width, thickness, and position on the substrate of the resistance heating element are the same as those in the first embodiment, and the other image heating apparatus and image forming apparatus are the same as those in the first embodiment.

  FIG. 10 is a schematic diagram of resistance temperature characteristics of the upstream resistance heating element 9 and the downstream resistance heating element 10 in the present embodiment. In the figure, the solid line indicates the resistance temperature characteristic of the upstream resistance heating element 9, and the broken line indicates the resistance temperature characteristic of the downstream resistance heating element 10. Table 2 below shows the temperature change of the resistance value and the heat generation ratio of the upstream and downstream.

Also in this embodiment, the temperature increase rate of the downstream resistance heating element at the time of abnormal temperature rise is higher than that of the upstream resistance heating element. Even in this case, it is possible to prevent cracking of the ceramic substrate of the heating element by disposing the thermal element 11 above the downstream resistance heating element 10.

<< Third Embodiment >>
FIG. 11 is a diagram illustrating a front view of a heating body and a circuit for performing energization control in the present embodiment. In the present embodiment, as shown in FIG. 11, the two resistance heating elements 12 and 13 are electrically connected in parallel. FIG. 11 also shows the back surface (sliding surface of the inflexible member) of the heating body 3, and the thermal element 11 is provided on the back surface in a portion corresponding to the downstream resistance heating element 13.

  In this embodiment, as the material of the resistance heating element, the upstream resistance heating element 12 is mixed with silver palladium and glass, and the downstream resistance heating element 13 is ruthenium oxide. Upstream, the resistance change rate of the resistance heating element 12 was +500 ppm / ° C., and the resistance change rate of the downstream resistance heating element 13 was −5000 ppm / ° C. These were formed by screen printing to a thickness of about 10 μm, a width of 1.1 mm, and a total length of 220 mm, both upstream and downstream.

  In this embodiment, similarly to the other embodiments, the heating body protrudes 2 mm upstream of the nip, and the upstream resistance heating element 12 of the heating body is located outside the nip. The downstream resistance heating element is located in the nip.

  FIG. 12 shows a schematic diagram of resistance temperature characteristics of the upstream resistance heating element 12 and the downstream resistance heating element 13 in the present embodiment. In FIG. 12, the solid line indicates the resistance temperature characteristic of the upstream resistance heating element 12, and the broken line indicates the resistance temperature characteristic of the downstream resistance heating element 13. In the start-up region from room temperature, the magnitude relationship between the resistance value R2 of the upstream resistance heating element 12 and the resistance value R1 of the downstream resistance heating element 13 is R1> R2. At the above temperature, R1 <R2.

  FIG. 13 shows the temperature distribution in the width direction of the heating element in the present embodiment. During normal use, the temperature distribution is substantially contrasted between the upstream heating element 12 and the downstream heating element 13, but the amount of heat generated by the downstream heating element 13 during an abnormal temperature increase in the case of a triac failure or the like. Is bigger. This is because when the temperature rises abnormally, the resistance difference between the upstream resistance heating element 12 and the downstream resistance heating element 13 becomes large, so that a very large current is generated compared to the upstream resistance heating element 12. By flowing into. This is because the temperature of the downstream resistance heating element 13 is increased more rapidly than the heat absorbed by the ceramic substrate and the surrounding members.

  In the heating device of this embodiment, when a runaway test was performed in which the triac was intentionally shorted to abnormally raise the temperature of the heating body, the thermoswitch 11 was turned off before the ceramic substrate cracked, and the heating device stopped heating. .

<< Fourth Embodiment >>
FIG. 14 is a diagram illustrating a front view of a heating body and a circuit that performs energization control in the present embodiment. In the present embodiment, as shown in FIG. 14, the two resistance heating elements 14 and 15 are electrically connected in parallel.

  In this embodiment, as the material of the resistance heating element, the upstream resistance heating element 14 is made of barium titanate having a Curie temperature of 200 ° C., and the downstream resistance heating element 15 is mixed with silver palladium and glass. These were formed by screen printing to a thickness of 10 μm, a width of 1.1 mm, and a total length of 220 mm in both upstream and downstream.

  Barium titanate used as the upstream resistance heating element 14 can shift the Curie temperature to a desired temperature by adding impurities. The Curie temperature of barium titanate is 120 ° C. In this embodiment, the Curie temperature is shifted to 200 ° C. by adding lead.

  FIG. 15 shows a schematic diagram of resistance temperature characteristics of the upstream resistance heating element 14 and the downstream resistance heating element 15 in the present embodiment. In FIG. 15, the solid line indicates the resistance temperature characteristic of the upstream resistance heating element 14, and the broken line indicates the resistance temperature characteristic of the downstream resistance heating element 15.

  The resistance value of silver palladium of the downstream resistance heating element 15 continues to rise gradually from room temperature to high temperature. On the other hand, although the resistance value of the barium titanate of the upstream resistance heating element 14 decreases as the temperature increases from normal temperature, the resistance value increases from about 200 ° C. which is the Curie temperature. Since the upstream resistance heating element 14 and the downstream resistance heating element 15 are electrically connected in parallel, the resistance value of the upstream resistance heating element 14 is higher than the resistance value of the downstream resistance heating element 15 at 200 ° C. or higher. It becomes a big value to the difference.

  In this case, the current flows only to the downstream resistance heating element 15 having a small resistance value. Therefore, since the upstream heating element 14 cannot receive power at 200 ° C. or more, the upstream resistance heating element 14 cannot be heated to 200 ° C. or more, and the temperature of the upstream resistance heating element 14 is around 200 ° C. Saturates (self-temperature control). Therefore, the temperature control temperature of the downstream heating element 15 and the Curie temperature of the upstream resistance heating element are adjusted to be the same.

  As a result, the upstream resistance heating element 14 is positioned outside the nip during normal use, and the upstream resistance heating element 14 is heated downstream from the nip even if the temperature is not controlled by the thermistor. It can be maintained at a temperature equivalent to the fixing temperature of the body 10.

  In the heating element as in the present embodiment, when the temperature rises abnormally, no current flows through the upstream resistance heating element 14 and almost all the electric power is input to the downstream resistance heating element 15, so Sometimes the temperature rise of the downstream resistance heating element becomes extremely large. As a result, as shown in FIG. 16, the temperature distribution in the width direction of the heating element is biased toward the downstream side.

  Also in the heating apparatus of this embodiment, when a runaway test was performed in which the TRIAC was intentionally shorted to abnormally raise the temperature of the heating body, the thermoswitch 11 was broken before the ceramic substrate was cracked, as in the other embodiments. The heating device stopped heating.

  As described above, by arranging the thermal element 11 close to or in contact with the heating element above the downstream resistance heating element while suppressing the problem of the reduction of the tailing phenomenon and the ceramic substrate cracking during normal use, It is possible to prevent the ceramic substrate from cracking during abnormal temperature rise.

(Modification)
In the above-described embodiments, two resistance heating elements electrically connected in series or in parallel have been described. However, the present invention is not limited to this, and two or more resistors electrically connected in series or in parallel. Any heating element may be used. That is, at least one of the resistance heating elements is located inside the nip portion N, and at least one of the resistance heating elements or a part thereof is preheated (preheated) on the upstream side of the nip portion N. ) To be located outside the nip portion N.

  For example, as shown in FIG. 18, a resistance heating element G is provided inside the nip N, and a resistance heating element F (a part of the nip N protrudes from the nip N) outside the nip N toward the upstream side of the nip. A resistance heating element E (all protruding from the nip portion N) may be provided. In this case, the resistance heating element G on the downstream side is electrically connected to the resistance heating element on the upstream side (here, a plurality of resistance heating elements are arranged in parallel or in series as the resistance heating elements F and E). Need only be connected in series or in parallel.

  Note that at least one of the upstream resistance heating elements F and E in FIG. 18 may be the self-temperature control type resistance heating element described in the fourth embodiment.

  In the embodiment described above, the resistance heating element provided outside the nip portion N is provided on the upstream side of the nip portion N. However, the present invention is not limited to this, as shown in FIG. You may provide in the downstream of the nip part N. FIG. In FIG. 19A, a resistance heating element J is provided on the inner side of the nip portion N, and a resistance heating element K is provided on the downstream side of the nip portion as the outer side of the nip portion N.

  The portion of the flexible member F (heated at the position Y) heated by the resistance heating element K on the downstream side of the nip portion N makes almost one turn on the premise that the entire circumference is shortened for compactness. Thus, the position X reaches the upstream side of the nip portion N. The conveyance system of the recording material P is set so that the elapsed time from the position Y to the position X is substantially equal to the time for transmitting the inside of the flexible member F and the recording material P is preheated (preheated) at the position X. If it is rare, the occurrence of tailing can be suppressed.

  With respect to the resistance heating element K on the downstream side of the nip portion, the whole may protrude from the nip portion N as shown in FIG. 19A, or a part of the resistance heating element K may protrude from the nip portion N as shown in FIG. It may protrude. Although a resistance heating element may be provided on the upstream side of the nip portion, in FIG. 14A, a supporting member L for extrapolating the flexible member F is configured instead of the resistance heating element.

  In the above-described embodiment, the resistance heating element is provided on the lower side of the substrate 31 (the side facing the nip portion N), but as shown in FIG. 20, the upper side of the substrate 31 (facing the nip portion N). It may be provided on the opposite side). In FIG. 20, reference numeral 11 denotes a protection of a glass coat, a fluororesin layer, etc. provided to satisfy the withstand voltage between the resistance heating elements 7 and 10 on the downstream side formed on the upper side of the substrate 31 and the temperature detecting element 5. Is a layer.

  In the above-described embodiment, the substrate 31 on which two or more resistance heating elements are arranged is made of ceramics. However, any material other than ceramics may be used as long as the material has similar properties such as thermal conductivity.

  In addition, you may take the structure which combines suitably the matter described in the 1st thru | or 4th embodiment and the modification within the scope of the present invention.

1 .... Stay, 2 .... Flexible member, 3 .... Heating body, 4 .... Pressure roller, 4a ..., Metal core,
4b ··· Elastic body layer, 4c · · Release layer, 5 · · Temperature sensing element, · · · Upstream resistance heating element, · · · Downstream resistance heating element, 11 · · · Thermal element, 21, 21 · · · Feeding Electrode, 24 ... CPU,
25 ... Triac, 26 ... AC power supply, 31 ... Substrate, N ... Nip, P ... Recording material, T ... Toner, a ... Recording material conveyance direction

Claims (10)

A nip portion is provided between the heating member and a pressure member that contacts and slides on one surface with the heating member and contacts the other surface with a recording material carrying an image. In the image heating apparatus that transmits heat of the heating body to the recording material through the flexible member by moving the flexible member and the recording material with respect to the heating body.
The heating body is
The board has two or more resistance heating elements,
At least one of the resistance heating elements is located inside the nip portion,
At least one of the resistance heating elements or a part thereof is located outside the nip portion for preheating the recording material upstream of the nip portion,
The resistance heating element located inside the nip part and the resistance heating element located outside the nip part are electrically connected in series or in parallel,
When the resistance heating element located inside the nip portion and the resistance heating element located outside the nip portion are connected in series, the resistance heating element located outside the nip portion is With negative resistance temperature characteristics,
When the resistance heating element located inside the nip part and the resistance heating element located outside the nip part are connected in parallel, the resistance heating element located outside the nip part is positive. With resistance temperature characteristics,
A temperature control system for controlling the temperature of the resistance heating element located inside the nip portion;
A thermal element disposed near or in contact with the heating body at a position corresponding to the inside of the nip portion, and shuts off power supply to the heating body when the heating body is abnormally heated. Image heating device.   The image heating apparatus according to claim 1, wherein the resistance heating element located outside the nip portion includes at least one of ruthenium oxide, carbon, and silicon carbide.   The image heating apparatus according to claim 1, wherein the resistance heating element located outside the nip portion includes a self-temperature control type.   The image heating apparatus according to claim 3, wherein the self-temperature control type resistance heating element located outside the nip portion is made of barium titanate.   5. The image heating according to claim 1, wherein the resistance heating element located outside the nip portion is located upstream in a conveyance direction of the recording material. apparatus.   5. The image heating according to claim 1, wherein the resistance heating element positioned outside the nip portion is positioned downstream of the recording material conveyance direction. 6. apparatus. At least one of the resistance heating element positioned inside the nip portion and the resistance heating element positioned outside the nip portion is positioned outside the resistance heating element positioned inside the nip portion and the nip portion. When the resistance heating element is connected in series, a plurality of the resistance heating elements arranged in parallel electrically,
When the resistance heating element positioned inside the nip portion and the resistance heating element positioned outside the nip portion are connected in parallel, the plurality of resistance heating elements arranged in series are electrically connected. The image heating apparatus according to claim 1, further comprising: an image heating apparatus according to claim 1;   The image heating apparatus according to claim 1, wherein the two or more resistance heating elements are provided on a side of the substrate facing the nip portion.   The image heating apparatus according to any one of claims 1 to 7, wherein the two or more resistance heating elements are provided on a side opposite to a side facing the nip portion of the substrate.   The image heating apparatus according to claim 1, wherein the substrate is a ceramic substrate.