1.  Field of the Invention
The present invention relates to an image fixing apparatus used for an image forming apparatus such as a printer, a copying machine and a facsimile. 
2.  Description of Related Art
A thermal roller type image fixing apparatus and a film heating type image fixing apparatus have hitherto been known as an image fixing apparatus in the image forming apparatus such as the copying machine and the printer. 
In particular, the film heating type image fixing apparatus is, as compared with the thermal roller type image fixing apparatus, effective as an energy saving/on-demand image fixing apparatus capable of restraining an electric power consumption as low as possible without supplying the electric power when in a standby mode. 
The image fixing apparatus includes, as a basic configuration, a heating member (heater) constructed of a ceramic substrate, a heat generating resistor (heat generation resistor), etc.,  a heating member support member for supporting the heating member, a flexible member sliding on the heating member, and a pressuring member press-fitted to the heating member through the flexible member and thus forming a fixing nip, wherein the fixing nip portion nips and conveys a recording material formed with a unfixed image, and the unfixed image is fixed as a permanent image on the recording material by the heat transferred from the heating member via the flexible member.  The flexible member involves using a film made of a heat resistive thin resin or a film made of a metal. 
FIG.  14 is an enlarged cross-sectional model view of the fixing nip portion in one example of the film heating type image fixing apparatus.  A ceramic heating member 73 formed as an elongate thin plate member is depicted in the drawing in FIG.  14, wherein its longitudinal direction is a vertical direction in FIG.  14.  The ceramic heating member 73 uses a ceramic substrate made of alumina etc.  as a heater substrate, and is a low heat-capacity linear heating member constructed of a ceramic substrate 73a, a heat generating resistor 73b so provided as to be formed along the longitudinal direction of the ceramic substrate on one-surface side of this ceramic substrate 73a, a surface protective layer 73c covering a heat generation resistor forming surface side of the ceramic substrate 73a and composed of a glass layer, and so on. 
The ceramic heating member 73 is fitted into the heating member support member (heater holder) 71 made of a heat resisting resin etc.  This heating member support member 71 is provided with a heating member fitting groove portion 711 for fitting the ceramic heating member 73, and the ceramic heating member 73 is fitted into this heating member fitting groove portion 711.  The side of the surface protective layer 73c of the ceramic heating member 73 is the surface side of the ceramic heating member 73, and the side of surface protective layer 73c of the surface thereof faces a nip portion that will be explained later on. 
The ceramic heating member 73 is disposed into a film 72 formed as a flexible member.  An elastic pressurizing roller 74 serving as a backup roller is disposed oh the side opposite to the ceramic heating member 73 with respect to the film 72.  The elastic pressurizing roller 74 has an elastic body layer 74b, and an outer peripheral surface of the elastic body layer is covered with a release layer 74c.  The surface of the ceramic heating member 73 supported on the heating member support member 71 and the pressurizing roller 74 cooperate to form a fixing nip portion N with the film 72 being nipped therebetween.  The film 72 is, as the pressurizing roller 74 is rotationally driven or by another film driving means, moved in a direction indicated by an arrowhead while its internal surface comes into contact with the surface of the ceramic heating member 73. 
The ceramic heating member 73 rises in temperature quickly on the whole by a heat dissipation by itself from the heat generating resistor due to electric conduction to the heat generating resistor 73b.  Then, a state of the temperature of the ceramic heating member 73 is detected by temperature detecting means (not shown), and an information of the temperature detected by the temperature detecting means is inputted to temperature control means (not shown) from the temperature detecting unit.  The temperature control unit controls the electric power supply to the heat generating resistor 73b so that the heating member temperature information inputted from the temperature detecting unit is kept at a predetermined fixing temperature, thus controlling the temperature of the ceramic heating member 73. 
A recording material P bearing an unfixed toner image t is passed through the nip portion N of a thus-temperature-controlled fixing unit, whereby the toner image t is fixed by heating onto the recording material P. The symbol A represents a recording material conveyance direction.  The recording material P passing through the fixing nip N is curvature-separated from the film surface and is then conveyed. 
In the film heating type image fixing apparatus, generally, the ceramic heating member 73 is supported by forming a heating member seating face on the heating member support member 71 and nipping the ceramic heating member 73 between this heating member seating face and the fixing nip portion N.
The following are specific configurations of the seating face. 
1) One configuration, as shown in FIG.  14, is that a bottom face of the heating member fitting groove portion 711 of the heating member support member 71 sustains the entire rear surface of the heating member. 
2) Another configuration, as shown in FIG.  15, is that the heating member seating face 71a sustains only short-directional upstream and downstream sides of the heating member in order to efficiently transfer the heat of the heat generating resistor 73b toward the fixing nip N by speeding up a rise in temperature of the ceramic heating member 73 itself, and the heat is cut off by providing an air gap portion 712 between two portions of heating member seating faces 71a.  
In the case of a printer that outputs the sheets of which the number is not so large per unit time, an amount of electric power supply to the heat generating resistor is small, and hence the configuration shown in FIG.  15 is effective especially in quickly starting up the heating member in an image-fixable state. 
In the case of a printer that outputs a large number of sheets per unit time, however, the amount of electric power supply to the heat generating resistor increases.  In this type of printer outputting the large number of sheets per unit time, if the electric power supply to the heat generating resistor abruptly increases, a temperature of a portion provided with the heat generating resistor of the heating member sharply rises, and there is a large temperature difference from a portion provided with none of the heat generating resistor of the heating member.  It then proves that a stress is applied to the substrate of the heating member due to this temperature difference, with the result that heating member is broken. 
For example, if the temperature of the heating member excessively rises due to (thermal) runaway (unable control) of the image fixing apparatus as in the case of a fault of a TRIAC that controls an adjustment of the temperature of the heating member, there is a possibility that the ceramic substrate might be broken before a temperature over-rise preventive element (a temperature fuse, a thermo switch) abutting on the heating member operates. 
In the case of the configurations of the conventional heating member, the heating member seating face and the fixing nip in FIGS.  14 and 15, the heat from the heat generating resistor 73b transfers toward the fixing nip N, the ceramic heating member 73 itself, and the heating member support member 71 via the heating member seating face 71a.  In any one of the configurations in FIGS.  14 and 15, however, the heat transfers to the heating member support member 71 from a heating member edge portion provided with none of the heat generating resistor 73b, and consequently there increases a temperature difference from a portion (resistor forming area) Wh provided with the heat generating resistor 73b.  Particularly in the configuration in FIG.  15, the heating member portion corresponding to the air gap portion 712 quickly rises in temperature, however, the heat at a portion abutting on the seating face 71a escapes to the heating member support member 71, with the result that a temperature rising speed slows down.  Therefore, the temperature difference in an interior of the heating member becomes much larger, and a margin to a damage to the heating member is small because of a large thermal stress.  The configuration in FIG.  14 shows a smaller temperature difference within the heating member than in the configuration in FIG.  15, however, the large temperature difference between the resistor forming area Wh and the heat generating resistor non-forming area is still easy to occur. 
Further, as disclosed in Japanese Patent Application Laid-Open No.  H10-144453 and Japanese Patent Application Laid-Open No.  H10-125450, there is proposed a method, wherein as the configuration of the seating face of the heating member support member, a contact area between the heating member and the heating member support member is set as small as possible, there is reduced a temperature difference caused between the area formed with the heat generating resistor of the heating member and the area provided with no heat generating resistor, a margin to the damage to the heating member when running away is increased by reducing the thermal stress applied to within the heating member.  If the area of the seating face is decreased, the heat transfer to the heating member support member from the heating member is restrained, and therefore the image fixing apparatus can be also swiftly started up to the image-fixable temperature.  FIG.  16 shows the seating face configuration disclosed in Japanese Patent Application Laid-Open No.  H10-125450.  To be specific, the contact area between the ceramic heating member 73 and the heating member support member 71 is reduced to the greatest possible degree by decreasing a total area of a heating member seating face 71a receiving the ceramic heating member 73. 
In the image fixing apparatus having the configuration as shown in FIG.  16, however, in the case of feeding a sheet through (which will hereinafter be referred to as a small-sized sheet), which is narrow in width for a width, in a longitudinal direction (a direction orthogonal to a recording material conveyance direction), of the heat generating resistor as in an envelope and a postal card, a temperature of a sheet non-feeding portion that will be mentioned later on rises more greatly than in the conventional configurations in FIGS.  14 and 15. 
In the longitudinal direction of the heating member, the heat in the area through which the sheet passes is absorbed by the sheet, and hence the electric power is supplied to the heating member (precisely to the heat generating resistor) to compensate for an amount of heat radiation.  A fixing temperature is maintained in the sheet feeding area of the heating member under this type of control.  The heat is not, however, absorbed by the sheet in an area through which the sheet does not pass, so that the heat in this area rises higher than the fixing temperature.  Such a phenomenon is called a sheet non-feeding portion temperature rise.  If an excessive sheet non-feeding portion temperature rise occurs, durability of components building up the image fixing apparatus decline. 
As described above, the seating face configuration illustrated in FIG.  16 enables, the stress applied to the heating member when the sheet is not fed through the fixing nip portion, to be restrained to some extent. 
However, the rear surface of the heating member is rendered adiabatic by an air layer over a wide range, and hence a function of restraining the sheet non-feeding portion temperature rise when feeding the small-sized sheet through is smaller than in the configurations in FIGS.  14 and 15.  Further, in the configuration in FIG.  16, the heat is harder to escape to the heating member support member from the heating member on the whole than in the conventional configurations in FIGS.  14 and 15, however, there is still the large internal temperature difference within the heating member between the resistor forming area Wh of the heating member and the heat generating resistor non-forming area, and the margin to the thermal stress described above is also still insufficient.