Patent Publication Number: US-7215899-B2

Title: Image forming apparatus having temperature sensing element for sensing temperature of recording material

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus such as a copying machine or printer using electrophotography or electrostatic recording. In particular, it relates to an image forming apparatus having temperature sensing means for sensing the temperature of a recording material after fusing. 
   2. Description of the Related Art 
   An image forming apparatus, such as a copying machine or printer, using electrophotography or electrostatic recording is provided with a fuser for heat-fixing or fusing a toner image onto a recording material. In this case, various techniques are employed to increase the fixability of the image. 
   One of such techniques, there is proposed a technique for sensing the temperature of a recording material after fusing, and feeding it back so that it will be brought close to a control target temperature. For example, see Japanese patent application laid-open No. H01-150185 (1989), Japanese patent utility model laid-open No. H01-160473 (1989), and Japanese patent application laid-open Nos. H03-53276 (1991), H04-181250 (1992), H06-308854 (1994), H07-230231 (1995), H07-239647 (1995), H10-161468 (1998), 2000-66461, 2001-13816, 2002-23555, 2002-214961, and 2003-29485. 
   For example,  FIG. 9  shows an example of a fusing device using a non-contact sensor for sensing the temperature of a recording material after fusing. In such a fusing device, a non-contact sensor  20  such as an infrared sensor is placed downstream of a fixing area or nip so that it measures the temperature of the recording material in a non-contact manner. 
     FIG. 10  shows an example of a fusing device using a contact type sensor for sensing the temperature of a recording material after fusing. In such a fusing device, a temperature sensor  18  such as a thermistor is placed downstream of a fixing nip, and an opposed member  19  such as a rubber roller is placed to face the temperature sensor, so that the recording material is nipped between the temperature sensor and the opposed member, allowing the temperature sensor to measure the temperature of the recording material. 
   In the structure for sensing the temperature of the recording material to feed back the sensed temperature, temperature sensing accuracy is crucial. 
   Upon fusing the recording material, since the moisture contained in the recording material is also heated, water vapor is produced from the surface of the recording material. In the case of temperature detection using the non-contact sensor, the water vapor fogs the surface of the non-contact sensor, and this makes it difficult to detect the temperature of the recording material accurately. 
   On the other hand, in the method of bringing the opposed member into contact with the temperature sensor so that the temperature sensor can detect the temperature of the recording material nipped between the temperature sensor and the opposed member, the opposed member draws heat from the recording material, and this also make it difficult to detect the temperature of the recording material accurately. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problems, and it is an object thereof to provide an image forming apparatus capable of setting fixing conditions irrespective of the kind of recording material. 
   It is another object of the present invention to provide an image forming apparatus having a high degree of accuracy of temperature sensing of a recording material. 
   It is still another object of the present invention to provide a heating device for an image forming apparatus. 
   An image forming apparatus according to the present invention includes image forming means for forming an image on a recording material, fusing means having a fixing nip area for nipping and conveying the recording material and provided for fusing an image onto the recording material, and temperature sensing means for sensing the temperature of the recording material on the downstream side of the fixing nip area in the moving direction of the recording material. In this structure, a temperature sensing part of the temperature sensing means is arranged to come into contact with the side opposite to the image side of the recording material at the time of one-sided printing without providing such an opposed member that comes into contact with the image side of the recording material at least in a position corresponding to the position of the temperature sensing part when the temperature sensing part is in contact with the recording material. 
   Further objects of the present invention will become more clearly apparent when the following description is read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view showing components in the vicinity of a temperature sensing part for a recording material according to a first embodiment of the present invention. 
       FIG. 2  is a perspective view showing components in the vicinity of a temperature sensing part for a recording material according to a second embodiment of the present invention. 
       FIG. 3  is a perspective view showing components in the vicinity of a temperature sensing part for a recording material according to a third embodiment of the present invention. 
       FIG. 4  is a cross sectional view showing components in the vicinity of a temperature sensing part for a recording material according to a fourth embodiment of the present invention, in which no recording material passes through the temperature sensing part. 
       FIG. 5  is a cross sectional view showing the components in the vicinity of the temperature sensing part for a recording material according to the fourth embodiment of the present invention, in which a recording material is passing through the temperature sensing part. 
       FIG. 6  is a cross sectional view showing components in the vicinity of a temperature sensing part for a recording material according to a fifth embodiment of the present invention, in which no recording material passes through the temperature sensing part. 
       FIG. 7  is a cross sectional view showing the components in the vicinity of the temperature sensing part for a recording material according to the fifth embodiment of the present invention, in which a recording material is passing through the temperature sensing part. 
       FIG. 8  is a cross sectional view of an electrophotographic printer as an example of an image forming apparatus according to the present invention. 
       FIG. 9  is a cross sectional view showing a conventional technique for sensing the temperature of a recording material using a non-contact temperature sensor. 
       FIG. 10  is a cross sectional view showing another conventional technique for sensing the temperature of a recording material nipped between a temperature sensor and an opposed roller. 
       FIG. 11  is a cross sectional view showing components in the vicinity of a temperature sensing part for a recording material according to a sixth embodiment of the present invention, in which no recording material passes through the temperature sensing part. 
       FIG. 12  is a cross sectional view showing the components in the vicinity of the temperature sensing part for a recording material according to the sixth embodiment of the present invention, in which a recording material is passing through the temperature sensing part. 
       FIG. 13  is a cross sectional view showing the position of a recording material conveying guide and a virtual line connecting a fixing nip area and a nip area between delivery rollers according to the sixth embodiment of the present invention. 
       FIG. 14  is a cross sectional view showing a state of sensing the temperature of a recording material according to the sixth embodiment of the present invention. 
       FIG. 15  is a perspective view of the components in the vicinity of the temperature sensing part for a recording material according to the sixth embodiment of the present invention. 
       FIG. 16  is a cross sectional view showing a state of sensing the temperature of a recording material according to a seventh embodiment of the present invention. 
       FIG. 17  is a perspective view of temperature sensing means, as seen from the upstream side of the recording material conveying direction, according to the seventh embodiment of the present invention. 
       FIG. 18  is a perspective view of the temperature sensing means, as seen from the downstream side of the recording material conveying direction, according to the seventh embodiment of the present invention. 
       FIG. 19  is a perspective view of temperature sensing means, as seen from the downstream side of the recording material conveying direction, according to an eighth embodiment of the present invention. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   (First Embodiment) 
     FIG. 8  is a schematic cross-sectional view of an electrophotographic printer as an example of an image forming apparatus to which the present invention is applied. 
   This printer is provided with a sheet feeding device consisting predominantly of a paper tray  1 , a sheet stacker  2 , and a feed roller  3 . In operation, sheets of recording material P are stacked on the sheet stacker  2  in the paper tray  1 , picked up by the feed roller  3  one by one from top to bottom, and fed by conveyance rollers  4 ,  5  into a resist section. The recording material is jogged in the resist section consisting of resist rollers  6 ,  7  to correct its conveyance direction, and fed into an image forming section (image forming means). 
   In the image forming section, a photosensitive drum  8 , a charger (not shown) arranged around the photosensitive drum  8 , a developing unit (not shown) for developing with toner a latent image formed on the photosensitive drum, and a cleaning unit (not shown) for removing toner remaining on the photosensitive drum are integrated into a toner cartridge  9  removable from the printer main body. A laser scanner unit  10  for writing an image onto the photosensitive drum  8  according to image information contains a laser light source (not shown), a laser deflecting mirror (polygon mirror)  11 , a motor for driving the deflecting mirror to rotate (not shown), etc. 
   When the image information is inputted, the printer scans with laser light L the surface of the photosensitive drum  8  charged to a predetermined potential by the charger to form an electrostatic latent image on the photosensitive drum  8 . The latent image is developed with toner as developer by development means, and the developed toner image is transferred by a transfer roller  12  from the photosensitive drum  8  to the recording material. 
   The recording material on which the toner image has been transferred is fed into fusing means having a heating unit  13  and a backup unit  14 , and the toner image on the recording material is fused. After that, the recording material is ejected to an output tray  17  through a delivery unit composed of an intermediate delivery roller  15 , a delivery roller  16 , etc. 
     FIG. 1  is a cross sectional view showing components in the vicinity of a fusing device and temperature sensing means for sensing the temperature of a recording material after fusing. 
   The printer of the embodiment is provided with a fusing device (on-demand fixing device) of a film heating type for heating paper through a film- or belt-shaped flexible sleeve (hereafter called the fixing film). However, the present invention is not limited to the image forming apparatus provided with such an on-demand fixing device. It is also applicable to an image forming apparatus provided with any other one of various fusing devices, such as a fusing device of a heat roller type in which paper is heated while being nipped and fed between a heating roller, controlled to maintain constant temperature, and a pressure roller having an elastic body layer and pressed against the heating roller. 
   As mentioned above, after the toner image formed in the image forming section is transferred onto the recording material, the recording material is fed into a fusing section. The fusing section consists predominantly of the heating unit  13  and the backup unit  14 . The front edge of the recording material is led through an entrance guide  21  to a pressure nip area (fixing nip area) N formed between the heating unit and the backup unit. 
   The heating unit  13  consists predominantly of a fixing film  22 , a heater (heating body)  23  brought into contact with the inner surface of the fixing film, a film guide member  25  holding the heater  23  while guiding the fixing film, and a metal stay for pressing the film guide member on the backup unit. The backup unit consists predominantly of a pressure roller  24 . When the end of the metal stay is forced onto the pressure roller by means of a coil spring or the like, pressure is exerted on the fixing nip area N. 
   A release layer is formed around the surface of the fixing film  22 . The fixing film is loosely fitted around the film guide member  25  having roughly a semicircular shape in cross-section. 
   The fixing film  22  preferably has a low heat capacity in order to enhance quick-start performance. For example, it is a heat-resistant resin film of 100 μm thick or less, preferably in the range between 20 μm and 60 μm, with a polyimide or PEEK base layer. Alternatively, it may be a metal film of a stainless steel or the like with a nickel electroformed base layer. Since the metal film has excellent thermal conductivity, it is enough for outstanding quick-start performance that the thickness is 150 μm or less. 
   The heating body  23  may be a ceramic heater made up of a heat resistor (resistor pattern) formed on a ceramic substrate as a heat source, which is supplied with power to generate heat. When power is applied through the resistor pattern, the resistor pattern generates heat and the temperature of the heater rises. This heating body is made up in such a manner that a paste resistor, made of silver and palladium, is formed by thick film printing on a substrate of alumina (aluminum 203) or aluminum nitride (AlN) to form the resistor pattern with desired resistance. A glass layer is further formed on the resistor pattern. The glass layer serves not only to protect the resistor pattern, but also as a sliding layer being in friction with the inner surface of the fixing film. A thermistor as a temperature sensing element is bonded on the backside of the resistor pattern of the substrate. The thermistor monitors temperature and input temperature information into a control circuit section (not shown). The control circuit section controls an Ac driver to control the amount of power from an AC power supply to the heating body (resistor pattern) so that the temperature sensed by the thermistor will maintain a set temperature. 
   The pressure roller  24  has an elastic layer of silicone rubber around a metal core made of iron or aluminum, and a PFA tube layer as a release layer around the elastic layer. The pressure roller is driven by a driving motor, not shown. 
   The fixing film  22  receives a driving force from the pressure roller  24  to rotate in a clockwise direction in  FIG. 1  according to the rotation of the pressure roller. The recording material carrying an unfixed toner image is nipped and fed in the fixing nip area N between the heating body  23  and the pressure roller  24  through the fixing film  22 . The toner image is fused onto the recording material as it passes through the fixing nip area N. 
   In other words, when the recording material passes through the fixing nip area N, thermal energy is transferred from the heating body to the recording material through the fixing film, so that the unfixed toner image on the recording material is fused and fixed. The recording material P that has passed through the fixing nip area N and been separated from the fixing film is then fed to a delivery section by a delivery roller pair (conveyance means)  113 . 
   The image forming apparatus of the embodiment has only a single-sided printing function. However, regardless of whether the image forming apparatus has a double-sided or single-sided printing function, the temperature sensing part of the temperature sensing means of the present invention is placed to contact the side (non-printed side) opposite to the printed side of the recording material in the single-sided printing mode so that the temperature sensing part will contact the recording material in an interval between the fixing nip area and conveyance means closest to the fixing nip area in the downstream side of the moving direction of the recording material. The conveyance means is driven by a drive source. 
   There are two advantages of detecting temperature on the non-printed side of the recording material. One advantage is that a heat transmit plate (hereinafter called a “heat collector plate”) resists the adhesion of toner because the recording material contacts the heat collector plate on the backside of the toner fixed surface at the time of normal one-sided printing. In other words, there is no danger of reducing temperature sensing accuracy due to adhesion of toner onto the heat collector plate. The other advantage is that the kind of recording material can be estimated from the sensed temperature, that is, from a difference in heat conductivity from the printed side to the non-printed side according to the kind of recording material, because thermal energy is transferred from the printed side and the temperature is sensed on the non-printed side. For example, the temperature on the non-printed side of a thin recording material is higher than that of a thick recording material. Therefore, it can be determined from the difference in temperature that the recording material is of a thin type when the sensed temperature is higher than a reference temperature, and that the recording material is of a thick type when the sensed temperature is lower than a reference temperature. This method of sensing the temperature of a recording material is particularly effective in the structure of a fuser having a heating element only on one side (print side in the embodiment) of the recording material, that is, no heating element on the other side (non-printed side) in this case. 
   (Structure of Temperature Sensing Means) 
   In  FIG. 1 , a fixing/delivery guide (recording material guiding member)  103  that forms part of the conveyance path for the recording material is provided between the fixing nip area N and a nip area  102  formed between the delivery roller pair (conveyance means closest to the fixing nip area N). One of the delivery roller pair is driven by a motor, not shown. The fixing/delivery guide  103  is made of a material having high heat resistance such as PBT or PET. A conveyance side  103   a  of the fixing/delivery guide  103  is arranged below a virtual line L connecting an end  101  of the fixing nip area N on the downstream side of the moving direction of the recording material and the nip area  102  between the delivery roller pair. The conveyance speed of the recording material between the delivery roller pair is set higher than that in the fixing nip area. In other words, when the recording material is conveyed while being nipped in both the fixing nip area N and the delivery roller nip area  102 , the recording material travels in this section approximately along the straight line L connecting both nips. 
   A heat collector plate  104  made from a thin aluminum or stainless steel plate of about 0.1 mm thick and having low heat capacity is fixed on part of the conveyance side of the fixing/delivery guide  103  along the direction perpendicular to the conveyance direction of the recording material (that is, along the width of the recording material). At least part of the heat collector plate  104  projects from the virtual straight line L so that the projecting part (temperature sensing part) will come into direct contact with the recording material when the recording material passes through the part. In other words, the temperature sensing part is located on the opposite side of the recording material guiding member relative to the virtual straight line connecting the fixing nip area N and the nip area  102  of the conveyance means. Thus, since the heat collector plate  104  has low heat capacity and is brought into direct contact with the recording material, the temperature of the heat collector plate  104  can be made almost equal to the temperature of the recording material in a short time. 
   A highly responsive temperature sensor  105  such as a thermistor is fixed on the backside of the heat collector plate  104 , for example, by adhesive bonding. As also shown in  FIG. 1 , there is no member (opposed member) contacting the recording material in an area above heat collector plate  104 , that is, in an area corresponding to at least the temperature sensing part above the printed side of the recording material that is in direct contact with the temperature sensing part of the heat collector plate  104 . When the recording material after fusing is conveyed from a fusing device  106 , the non-printed side of the recording material comes into contact with the heat collector plate  104 , causing the heat collector plate  104  to draw heat from the recording material. The heat is conducted into and sensed by the temperature sensor  105  on the backside of the heat collector plate  104 , thus detecting the temperature of the recording material. In this case, since the temperature sensor  105  is mounted directly underneath the position (temperature sensing part) in which the heat collector plate  104  contacts the recording material, the influence of the temperature gradient in the heat collector plate  104  can be minimized to create such a temperature sensing state that is nearly equivalent to the case where the temperature sensor comes into direct contact with the recording material, thus increasing temperature sensing accuracy. Further, since there is no member (opposed member) contacting the recording material in an area corresponding to at least the temperature sensing part above the printed side of the recording material that is in direct contact with the temperature sensing part of the heat collector plate  104 , the heat accumulated in the recording material is difficult to escape except to the heat collector plate  104 , thus achieving high temperature sensing accuracy. The structure in which there is no member (opposed member) contacting the recording material in an area corresponding to at least the temperature sensing part above the printed side of the recording material that is in direct contact with the temperature sensing part of the heat collector plate  104  is common to all other embodiments to be described later. In addition, the use of the metal material for the slide member (heat collector plate) over which the recording material slides makes it possible to prevent the slide member from being worn away, and hence to improve durability. 
   The temperature sensor  105  is an element, typified by the thermistor, which varies its resistance with temperature; it is encapsulated in glass in such a state that dumet wires are thermally bonded to a thermistor chip to connect electrodes. The other ends of the dumet wires are connected to a control circuit section (not shown) so that temperature information detected by the thermistor will be transmitted to the control circuit section. For example, based on the sensed temperature information, the image forming apparatus sets the control temperature of the heater to the optimum temperature for the kind of the recording material. 
   In order to reduce the heat capacity of the heat collector plate  104 , it is preferable to minimize the dimensions of the heat collector plate  104  both in the conveyance direction of the recording material and in a direction orthogonal to the conveyance direction and nearly parallel to the width of the recording material. Further, in the embodiment, since the metal heat collector plate  104  with low heat capacity is provided inside the fixing/delivery guide  103  made of plastic having low thermal conductivity, not only can the heat capacity be reduced, but also the thermal insulation can be increased, thereby increasing the responsiveness of the temperature sensor  105 . 
   (Second Embodiment) 
   A second embodiment will be described with reference to  FIG. 2 . In this embodiment, the surface geometries of the fixing/delivery guide  103  are altered so that the vicinity of the heat collector plate  104  will be separated as far as possible from the recording material being conveyed. This makes is possible to sense temperature with a higher degree of accuracy.  FIG. 2  is a perspective view of a fixing/delivery guide  103  having a step height to make the vicinity of the heat collector plate  104  apart from both sides of the paper guiding surface of the fixing/delivery guide  103 . In this embodiment, a step height  107  is formed to prevent the influence of heat from any components other than the heat collector plate  104  on the temperature of the heat collector plate  104  and the temperature of the recording material in the vicinity of the heat collector plate  104 . 
   The recording material is brought into contact with the heat collector plate  104  in such a local area, further improving temperature sensor responsiveness. Like in the first embodiment, this embodiment has no opposed member on the opposite side of the heat collector plate  104 . In this regard, this embodiment also has excellent temperature sensing accuracy, compared to the conventional examples in which there is an opposed member such as a roller cooperating with the temperature sensor to nip and feed the recording material. 
   There is a structural problem not only in this embodiment but also in the first embodiment and the other embodiments to be described later. When both-sided printing is performed on an image forming apparatus having a both-sided printing function, since the heat collector plate  104  comes into contact with a toner image on the first side of the recording material at the time of printing of the second side, there is apprehension that toner will adhere to the surface of the heat collector plate  104 . To avoid this, the surface of the heat collector plate  104  may be coated with Teflon™, or UV coating may be applied to the surface of the heat collector plate  104  to such an extend that it does not affect the heat conductivity of the heat collector plate  104 . The surface of the heat collector plate  104  may also be coated with PI (polyimide). 
   (Third Embodiment) 
   A third embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a perspective view of a fixing/delivery guide  103  having a rectangular depressed portion in the vicinity of the heat collector plate  104 . In this embodiment, a rectangular depressed portion  108  is provided to prevent the influence of heat from any components other than the heat collector plate  104  on the temperature of the heat collector plate  104  and the temperature of the recording material in the vicinity of the heat collector plate  104 . 
   (Fourth Embodiment) 
   A fourth embodiment will next be described with reference to  FIGS. 4 and 5 . 
   As mentioned above, the conveyance speed of the recording material between the delivery roller pair  113  is set higher than that in the fixing nip area. Therefore, the friction resistance of the heat collector plate  104  to the recording material may become large enough to damage the recording material depending on the speed setting or the kind of recording material. To avoid this, this embodiment illustrates an example of a structure for retracting the heat collector plate  104  attached to the fixing/delivery guide  103  when it receives a force from the recording material.  FIG. 4  shows a home position and  FIG. 5  shows a temperature sensing position. In  FIG. 4 , the heat collector plate  104  is formed integrally with a slide member (moving member)  109 , which can move up and down along a slide guide part  103   b  of the fixing delivery guide  103 . The slide member  109  is always forced upward by means of a spring  110 , and the heat collector plate  104  is retained by a stopper (not shown) in a position as shown in  FIG. 4 . Then, as shown in  FIG. 5 , when the heat collector plate  104  is pressed down by a recording material  111 , the heat collector plate  104  is retracted downward against the force of the spring  110 . This makes it possible to reduce the friction resistance of the heat collector plate  104  to the recording material  111 . Also in this embodiment, the temperature sensing part of the temperature sensing means is located on the opposite side of the recording material guiding member relative to the virtual straight line connecting the fixing nip area of the fusing means and the nip area of the conveyance means at least before the front edge of the recording material exits from the fixing nip area. This improves temperature sensing accuracy. 
   (Fifth Embodiment) 
   A fifth embodiment will next be described with reference to  FIGS. 6 and 7 . 
     FIGS. 6 and 7  illustrate another example of the structure for retracting the heat collector plate  104  attached to the fixing/delivery guide  103  when it receives a force from the recording material.  FIG. 6  shows a home position and  FIG. 7  shows a temperature sensing position. In  FIG. 6 , the heat collector plate  104  is fixed by a mounting member  112  at one end to a heat collector plate mounting part  103   c  of the fixing/delivery guide  103 . Then, as shown in  FIG. 7 , when the heat collector plate  104  is pressed down by the recording material  111 , the heat collector plate  104  turns in the direction of arrow M due to its spring properties, thus making the heat collector plate  104  retractable. In this embodiment, the heat collector plate  104  itself also serves as a moving member. This structure also makes it possible to reduce the friction resistance of the heat collector plate  104  to the recording material  111 . 
   (Sixth Embodiment) 
   A sixth embodiment will next be described with reference to  FIGS. 11 to 15 . Members having the same functions as those in the above-mentioned embodiments are given the same reference numerals. 
   A fixing/delivery guide  28  is provided between the fixing nip area N and the nip area of the delivery roller pair (conveyance means). The fixing/delivery guide  28  forms a conveyance path for a recording-material. The fixing/delivery guide is made of a material having high heat resistance such as PBT or PET. The conveyance surface of the fixing/delivery guide is arranged below a virtual straight line A (see  FIG. 13 ) connecting the fixing nip area and the delivery roller nip area. The conveyance speed of the recording-material between the delivery roller pair is set higher than that in the fixing nip area N. In other words, when the recording material is conveyed while being nipped in both the fixing nip area and the delivery roller nip area, the recording material travels without being in direct contact with the conveyance surface of the fixing/delivery guide. 
   The fixing/delivery guide  28  is provided with a moving member  29  one end of which is fixed to the apparatus main body. When coming into contact with the recording material, the moving member is bent by the pressing force of the recording material ( FIG. 12 ), while when not being in contact with the recording material, it is at the home position ( FIG. 11 ). The moving member is made from a thin aluminum or stainless steel plate of about 0.1 mm thick having spring properties and low heat capacity. 
   A heat collector plate  31  of the moving member  29  is so arranged that when the moving member  29  is at the home position, the heat collector plate  31  is seated above the virtual straight line A connecting the fixing nip and the delivery roller nip, while when the recording material passes, it comes into a direct contact with the recording material. In other words, the temperature sensing part of the temperature sensing means is located on the opposite side of the recording material guiding member relative to the virtual straight line connecting the fixing nip area of the fusing means and the nip area of the conveyance means at least before the front edge of the recording material exits from the fixing nip area. Thus, since the heat collector plate has low heat capacity and is brought into direct contact with the recording material, the temperature of the heat collector plate can be made almost equal to the temperature of the recording material in a short time. 
   When both-sided printing is performed, since the moving member comes into contact with a toner image on the first side of the recording material at the time of printing of the second side, there is apprehension that toner will adhere to the surface of the heat collector plate. To avoid this, the surface of the heat collector plate may be coated with Teflon™, or UV coating may be applied to the surface of the heat collector plate to such an extend that it does not affect the heat conductivity of the heat collector plate. The surface of the heat collector plate may also be coated with PI (polyimide). 
   A highly responsive temperature sensor  32  such as a thermistor is attached on the backside of the tip of the heat collector plate  31 , for example, by adhesive bonding ( FIG. 14 ). When the recording material after fusing is conveyed from the fusing device, since the recording material strikes the moving member to bend the moving member, the non-printed side of the recording material comes into contact with the heat collector plate, causing the heat collector plate to draw heat from the recording material. At this moment, the heat is transmitted to the temperature sensor on the backside to allow the temperature sensor to detect the temperature of the recording material. In this case, since the temperature sensor is mounted directly underneath the position in which the heat collector plate contacts the recording material, the influence of the temperature gradient in the heat collector plate can be minimized to increase the accuracy of sensing the temperature of the recording material. In addition, use of a metal material for a slide part of the moving member over which the recording material slides makes it possible to prevent the slide part from being worn away, and hence to improve durability. 
   The thermistor (the temperature sensor) is an element which varies its resistance with temperature; it is encapsulated in glass in such a state that dumet wires  33  are thermally bonded to a thermistor chip to connect electrodes. The other ends of the dumet wires are connected to a control circuit section so that temperature information detected by the thermistor will be transmitted to the control circuit section. 
   The vicinity of the moving member will be further described with reference to  FIG. 15 . A delivery roller pair (conveyance means) has a delivery rubber roller  26  driven by a driving motor to rotate, and a delivery roller  27  driven by the rotation of the delivery rubber roller  26 . The fixing/delivery guide  28  has a large clearance  36  in such a position that the moving member  29  turns to prevent the recording material from coming into contact with the paper guiding surface of the fixing/delivery guide in the vicinity of the area where the moving member comes into contact with the recording material. This makes it difficult for heat in the vicinity of the heat collector part to escape to the fixing/delivery guide, increasing the accuracy of sensing the temperature of the recording material. Further, as shown in  FIGS. 12 and 14 , the temperature sensing part of the moving member is set in such a manner that when the recording material is pushing down the moving member (temperature sensing position), the temperature sensing part comes to almost the same position as the nip position between the delivery rubber roller  26  and the driven delivery roller  27  in the passing direction of the recording material. In such a structure, when the moving member  29  is at the temperature sensing position, the position of the moving member  29  to force the recording material comes to almost the same position as the nip position between the delivery rubber roller  26  and the driven delivery roller  27  in the conveyance direction of the recording material, thereby preventing the recording material from being distorted by the force of the moving member  29 . Thus, the prevention of distortion of the recording material can result in preventing the recording material from getting loosened from the temperature sensing part. This structure is effective in improving temperature sensing accuracy. 
   (Seventh Embodiment) 
     FIG. 16  is a cross sectional view showing components in the vicinity of a temperature sensing part for a recording material in the image forming apparatus according to a seventh embodiment of the present invention.  FIG. 17  is a perspective view of a moving member as seen from the upstream side of the moving direction of the recording material.  FIG. 18  is a perspective view of the moving member as seen from the downstream side of the moving direction of the recording material. The following describes only the features of this embodiment. Since the other components are the same as those in the sixth embodiment, the description thereof will be omitted. 
   A moving member  29  of the embodiment is made up by integrating a heat collector plate  31 , which is a thin plate of about 0.1 mm thick (made of aluminum or stainless steel with low heat capacity), into a plastic substrate by outsert molding or the like. As will be described later, thermistor electrodes  34  are also molded integrally with the moving member  29 . The electrodes  34  serve to force the moving member to move from the temperature sensing position to the home position. 
   Like in the fourth to sixth embodiments, when the moving member  29  is at the home position, the heat collector plate  31  is seated above the virtual straight line connecting the fixing nip area and the delivery roller nip area. The front edge of the recording material that has passed through the fixing nip area comes first into contact with the plastic part of the moving member. Then the recording material progresses toward the downstream side and pushes the moving member to turn, bringing the heat collector plate  31  into contact with the non-printed side of the recording material. Thus, since the heat collector plate has low heat capacity and is brought into contact with the recording material, the temperature of the heat collector plate can be made almost equal to the temperature of the recording material in a short time. In order to reduce the heat capacity of the heat collector plate, it is preferable to minimize the dimensions of the heat collector plate both in the conveyance direction of the recording material and in a direction orthogonal to the conveyance direction and nearly parallel to the width of the recording material. Further, like in the sixth embodiment, the temperature sensing part of the moving member  29  is set in such a manner that when the recording material is pushing down the moving member (temperature sensing position), the temperature sensing part comes to almost the same position as the nip position between the delivery rubber roller and the driven delivery roller in the passing direction of the recording material. In such a structure, when the moving member  29  is at the temperature sensing position, the position of the moving member  29  to force the recording material comes to almost the same position as the nip position between the delivery rubber roller and the driven delivery roller in the conveyance direction of the recording material, thereby preventing the recording material from being distorted by the force of the moving member  29 . Thus, the prevention of distortion of the recording material can result in preventing the recording material from getting loosened from the temperature sensing part. This structure is effective in improving temperature sensing accuracy. 
   A highly responsive temperature sensor  32  such as a thermistor is attached on the backside of the tip of the heat collector plate  31 , for example, by adhesive bonding. When the recording material after fusing is conveyed from the fusing device, since the recording material pushes the moving member to turn, the heat collector plate  31  comes into contact with the non-printed side of the recording material P, causing the heat collector plate to draw heat from the recording material. At this moment, the heat is transmitted to the temperature sensor  32  on the backside to allow the temperature sensor  32  to detect the temperature of the recording material. In this case, since the temperature sensor is mounted directly underneath the position in which the heat collector plate contacts the recording material when the moving member turns (to the temperature sensing position), the influence of the temperature gradient in the heat collector plate can be minimized to increase the accuracy of sensing the temperature of the recording material. In addition, the use of the metal material for a slide part of the moving member over which the recording material slides makes it possible to prevent the slide part from being worn away, and hence to improve durability. 
   The thermistor is an element which varies its resistance with temperature; it is encapsulated in glass in such a state that dumet wires  33  are thermally bonded to a thermistor chip to connect electrodes. As mentioned above, the moving member  29  is made up by integrating the two electrodes  34 , made of metal such as stainless steel, into the plastic part by outsert molding or the like ( FIGS. 17 and 18 ). The dumet wires  33  are welded to the two electrodes  34 , respectively. These electrodes  34  are then connected to the control circuit section so that temperature information detected by the thermistor will be transmitted to the control circuit section. 
   The electrodes  34  are made from a thin sheet metal of about 0.1 mm such as a thin stainless steel plate. The electrodes  34  serve not only to transmit temperature information from the thermistor to the control circuit section, but also to force the moving member to move from the temperature sensing position to the home position. One end of each electrode  34  on the thermistor side is formed integrally with the plastic part of the moving member, and welded to the dumet wires  33  of the thermistor, with the other end connected to each terminal fixed to the fixing/delivery guide. When the moving member  29  turns from the home position toward the temperature sensing position, the electrodes  34  become twisted from their fixed terminal connection part as a pivotal point by the rotation of the moving member  29 . Then, the twist deformation causes the moving member  29  to returns to the home position. Since the electrodes  34  give an adequate turning force to the moving member  29  while receiving repeated stress from the movement of the moving member  29 , they take the shape of a crank as shown in  FIGS. 17 and 18  to prevent the occurrence of permanent deformation or rupture. 
   The following describes the tip portion of the moving member in more detail. As mentioned above, the heat collector plate  31  made of a material with low heat capacity is formed integrally with the plastic member  29  having low heat conductivity. A hollow space  35  is provided on the backside of the heat collector plate except for junctions with the plastic member. In other words, the backside of the heat collector plate  31  is exposed as seen from the downstream side of the moving direction of the recording material. This reduces the heat capacity of the heat collector plate and the vicinity, and hence makes it difficult for heat to be sensed by the temperature sensor  32  to escape, increasing the responsiveness of the temperature sensor. 
   (Eighth Embodiment) 
   An eighth embodiment will be described with reference to  FIG. 18 . In this embodiment, the dumet wires of the thermistor are directly connected to lead wires  37 . The lead wires are arranged along the rotating shaft of the moving member, and connected to the control circuit section so that temperature information detected by the thermistor will be transmitted to the control circuit section through the lead wires. A helical torsion spring  38  is used to force the moving member to turn. 
   Thus, in the embodiment, the helical torsion spring  38  is used to force the moving member to turn, and the lead wires for transmitting the output of the thermistor are arranged along the rotating shaft of the moving member. This makes it possible to realize a simple structure of a temperature sensor inexpensive enough to work with such a moving member that is operated relatively infrequently. 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the above-mentioned embodiments, and that various modifications may be made within the technical scope of the invention. 
   This application claims priority from Japanese Patent Application No. 2004-054638 filed Feb. 27, 2004 and Japanese Patent Application No. 2004-115596 filed Apr. 9, 2004, which are hereby incorporated by reference herein.