Fixing device with temperature sensing device

Abstract: In a device for sensing a temperature of a fixing roller, a thermistor is provided on a flat upper surface of a support member made of a foamed silicone material, with a heat conductive material provided between them. A heatproof protective sheet is wrapped around so that the thermistor is partially sunk into the support member by the tightness of the wrapping. The heat conductive material is thereby deformed to the thermistor's shape. At the same time, the top part of the thermistor is always protruded from the upper part of the support member. A holder keeps this construction in contact with the circumferential surface of the fixing roller.

Description: The present invention relates to a temperature sensing device that is provided on a fixing device of an image forming apparatus, for controlling fixing temperature. More specifically, it relates to a temperature sensing device according to the preamble of claim 1. Such temperature sensing device is disclosed by JP-A 59-44633. In general, temperature sensing in a fixing device is done by measuring temperature on the circumferential surface of a fixing roller. In order for the temperature sensing to be done accurately, temperature sensing devices of a type of direct pressure contact are used. The temperature sensing device mentioned above is one that is kept in contact with a rotating fixing roller with an optimum pressure. It is, for example, a temperature sensing device wherein a thermistor is supported as a temperature sensing element on a supporting member formed by elastic substance such as foamed silicone or the like, and all of them are covered by a protective sheet such as a polyimide film. This temperature sensing device is kept in pressure-contact with a circumferential surface of a fixing roller with a load of 50 - 150 gmmf, thereby the temperature on the roller surface is measured directly, and a heater built in the fixing roller is turned on or turned off by the measured temperature. Thus, the temperature may be controlled. When a temperature sensing element is supported directly by a supporting member, the temperature on the side of the temperature sensing element where it is in contact with a fixing roller is the same as that on the fixing roller surface, but the temperature on the other side being in contact with the supporting member is different because heat in the temperature sensing element is lost through the supporting member. For example, the temperature difference of 30°C through 40°C has been created for the measured temperature of about 190°C, making it impossible to measure accurately the temperature on the fixing roller surface. When setting the fixing temperature to 190°C, the actual controlled temperature on the fixing roller surface has been 200°C - 210°C that is higher than the temperature set on an apparatus. In addition to the above, the temperature difference mentioned above has tended to be increased by the change of pressure-contact condition of the supporting member caused by friction and vibration consequent on the rotation of the fixing roller. As one of means to overcome the temperature difference, there has been proposed a method wherein a thin metal sheet having a high thermal conductivity such as an aluminum foil is inserted between a temperature sensing element and a supporting member to avoid heat diffusion to the supporting member by means of heat-storing effect of the thin metal sheet, thereby it is possible to measure and control accurately the temperature on the fixing roller surface. With regard to the method mentioned above, there has been proposed methods in Japanese Utility Model Examined Publication No. 21315/1989 and Japanese Patent Examined Publication No. 51765/1989. In both cases, however, due to the structure wherein a temperature sensing element is embedded in a cutout and recess having a certain depth provided in advance on a supporting member, the top surface of the temperature sensing element is not always protruded above the supporting member because of the variation of the depth of the cutout and recess and of an embedded position, resulting in occasional inability of accurate measurement of temperature on the fixing roller surface, despite heat-storing effect enhanced by means of a thin metal sheet. The method suggested in Japanese Utility Model Publication Open to Public Inspection No. 63731/1988, likewise, is not common because it is expensive due to a specific thermistor used therein. The method suggested in Japanese Utility Model Examined Publication No.19612/1982 is one wherein a temperature sensing element is attached on a flexible thin layer to be in close-contact therewith and the thin layer is provided with a superficial layer having a lower coefficient of friction. The method, however, is not satisfactory on the point of thermal responding property. In the temperature sensing device as disclosed by the above mentioned JP-A 59-44633, an output terminal of a thermistor is connected electrically to lead wires in form of a copper foil pattern. The pattern is formed on the inside face of the protective sheet by plating or the like, i.e. the face towards the thermistor. Thereby, the lead wires are prevented from being broken. However, this temperature sensing device is not satisfactory on the point of temperature measuring accuracy, because the above mentioned heat loss again occurs in the thermistor. It is therefore object of the present invention to provide a temperature sensing device of the type mentioned at the beginning which is capable of coming in pressure-contact with a fixing roller constantly without being influenced by the accuracy of processing and assembling thereof and is capable of measuring temperature on the surface of the fixing roller accurately owing to reducing heat loss in the temperature sensing device. This object is achieved by a temperature sensing device as defined in claim 1. Advantageous embodiments are described in the subclaims. In the first constitution of a fixing device with a temperature sensing device for achieving the object mentioned above, the above-mentioned temperature sensing device comprises a temperature sensing element, a supporting member made of elastic material and a heat-conductive thin metal sheet which is inserted between the temperature sensing element and the supporting member with a part thereof protruded from the temperature sensing element, in a device for sensing temperature on a fixing roller of a fixing device that heats and fixes a toner image on an image-transfer material. In the constitution, when the temperature sensing device mentioned above is in pressure-contact with the fixing roller, the top of the temperature sensing element is kept in contact with the circumferential surface of the fixing roller through a protective sheet, while the bottom of the temperature sensing element is partially buried in the supporting member. With regard to the second constitution of a fixing device with a temperature sensing device, in a fixing device with a temperature sensing device comprising a temperature sensing element, a supporting member which is made of elastic material and supports the temperature sensing element and a protective sheet that covers the temperature sensing element and the supporting member, heat-conductive materials are coated on the supporting member including a portion where the supporting member is in contact with the temperature sensing element. With regard to the third constitution, in a fixing device with a temperature sensing element comprising a supporting member which is made of elastic material and supports the temperature sensing element and a protective sheet that covers the temperature sensing element and the supporting member, the supporting member is of a two-layer composition consisting of a heat-conductive elastic member and an adiabatic elastic member, and the temperature sensing element is held by the heat-conductive elastic member mentioned above. With regard to the fourth constitution, in a fixing device with a temperature sensing device comprising a temperature sensing element, a supporting member which is made of elastic material and supports the temperature sensing element, and a protective sheet that covers the temperature sensing element and the supporting member, the above-mentioned supporting member is kept in pressure-contact with the circumferential surface of the fixing roller at two points of the supporting member which are positioned at an upstream side and a downstream side in the direction of rotation of the fixing roller. In the fifth constitution of a fixing device with a temperature sensing device, it comprises a temperature sensing element, a supporting member which is made of elastic material and supports the temperature sensing element, and a protective sheet that covers the temperature sensing element and the supporting member, and the surface of a heat-sensing area of the temperature sensing element is formed to be a layer having high thermal conductivity. In the sixth constitution of a fixing device with a temperature sensing device, it comprises a temperature sensing element, a supporting member which is made of elastic material and supports the temperature sensing element, and a protective sheet that covers the temperature sensing element and the supporting member, and a sheet having high thermal conductivity is inserted respectively between the supporting member and the temperature sensing element and between the temperature sensing element and the protective sheet. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a), 1(b) and 1(c) are sectional views showing constitution of the first examples of a temperature sensing device of the invention, Fig. 2 is a graph showing the results of measurement conducted by the above-mentioned temperature sensing device, Fig. 3 is a perspective view showing how the above-mentioned temperature sensing device is mounted, Figs. 4(a), 4(b), 4(c) and 4(d) are sectional views showing constitution of the second examples of a temperature sensing device of the invention, Fig. 5(a), (b), (c) and (d) are sectional views showing constitution of the third example of a temperature sensing device of the invention, Fig. 6 is a sectional view showing constitution of the fourth example of a temperature sensing device of the invention, Figs. 7(a), 7(b) and 7(c) are sectional views showing the fifth examples of a temperature sensing device of the invention, and Figs. 8(a), 8(b) and 8(c) are sectional views showing the sixth examples of a temperature sensing device of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The first example of the invention will be shown in Figs. 1(a) - 1(c) and in Fig. 3. Basic constitution of a temperature of the invention is shown in Fig. 1(a) wherein the numeral 1 represents a thermistor that is a temperature sensing element, the numeral 2 represents a flat and thin metal sheet that is heat-conductive, 3 is an elastic supporting member whose top is flat and 4 is a metallic holder that holds the supporting member 3. Thin metal sheets having heat conductivity of λ = 100W/m°C or more are acceptable as the thin metal sheet 2, and an aluminum foil, a copper foil, a berylium foil, and a magnesium foil are appropriately used as a preferable one, and the thickness of an aluminum foil to be used is 20 - 30 µm. For the supporting member 3, foamed silicone which has a broad elastic area and high thermal resistance is used. Both the thermistor 1 and the supporting member 3 having the thin metal sheet 2 between them as shown in the figure and the metallic holder 4 are wrapped with heat-resisting protective sheet 5 such as a polyimide film together to be united. As shown in Fig. 1(b) and Fig. 1(c) showing a sectional view taken on arrowed line A-A in Fig. 1(b), therefore, the thermistor 1 is caused by the tension based on wrapping by means of the protective sheet 5 to be buried partly in the supporting member 3, forming a hemispherical concave on the top face of the supporting member 3. Due to the foregoing, the thin metal sheet 2 in a foil shape is transformed automatically into a hemispherical shape following the shape of the thermistor 1 to be positioned behind the thermistor, and both sides of the thin metal sheet remain at the top surface and come in contact with the fixing roller 10 under the pressure-contact condition. Therefore, the high heat-storing effect can be achieved. Further, since the thermistor 1 is held so that a part thereof is protruded from the top surface of the supporting member 3 by reaction force caused by elasticity of the supporting member 3, the thermistor 1 can be kept in pressure-contact with the surface of fixing roller 10 maintaining the firm contact condition despite dispersion of thermistor 1 itself and erroneous positioning thereof. In Fig. 2, results of experiments show the relation between temperatures (shown with solid lines) on the surface of a fixing roller controlled by a conventional temperature sensing device wherein no thin metal sheet or no heat-conductive coated layer 102 shown in the second example described later is used or no sheet-shaped cover 202 is used for wrapping the temperature sensing device and temperatures (shown with broken lines) controlled by the temperature sensing device of the invention. According to the results of experiments, when the temperature was controlled by the former temperature sensing device for the temperature on a fixing roller set to 190°C ± 5°C, the temperature on the surface of an experimental roller was 230°C - 240°C for overshooting and 200°C - 210°C for the controlled temperature, showing the controlled temperature exceeding by far the set temperature, while when the temperature was controlled by the latter, the temperature was 200°C ± 5°C for overshooting and 194°C ± 5°C for the controlled temperature, showing the controlled temperature extremely close to the above-mentioned set temperature. The temperature sensing device of the invention may be attached rotatably by connecting the metallic holder 4 to bracket 6 fixed in a fixing device through a through type shaft 7. The metallic holder 4 supported on the bracket 6 is urged clockwise by torsion spring 8 with a fulcrum of the through type shaft 7 so that it may bring the thermistor 1 in pressure-contact with the surface of the fixing roller 10. Urging force of the torsion spring 8 is adjusted so that the load for pressure-contact of thermistor 1 on fixing roller 10 may be within a range from 50 gf to 100 gf. When fixing roller 10 is removed on the occasion such as a maintenance, stopper 4A that is united with metallic holder 4 hits the bracket 6 to avoid rotation of the temperature sensing device exceeding the necessary amount. Other examples of the invention will be explained as follows, referring to Figs. 4(a) - 4(d), Figs. 5(a) - 5(d) and Fig. 6. Figs. 4(a) through 4(d) show basic constitution of the second example of a temperature sensing device, and the numeral 1 represents a thermistor that is a temperature sensing element, 3 represents an elastic supporting member, 102 represents a coated layer of heat-conductive material coated on the portion where the thermistor 1 is in contact with the supporting member 3, and 4 represents a metallic holder that holds the supporting member 3. For the supporting member 3, foamed silicone material having a broad elastic area and high thermal resistance is used, and as the heat-conductive material, on the other hand, Dotite (trade name) is appropriate, and it is coated to be a thin layer as shown in Fig. 4(a), or it is coated in a layer shape on supporting member 3 including the portion where temperature sensing element 1 is in contact with supporting member 3 partially as shown in Fig. 4(b). Both the thermistor 1 and the supporting member 3 having heat-conductive coated layer 102 between them as shown in the figure and the metallic holder 4 are wrapped with heat-resisting protective sheet 5 such as a polyimide film together to be united. As shown in Fig. 4(c) and Fig. 4(d) showing a sectional view taken on arrowed line A-A in Fig. 1(b), therefore, the thermistor 1 is caused by the tension based on wrapping by means of the protective sheet 5 to be buried partly in the supporting member 3, forming a hemispherical concave on the top face of the supporting member 3. Due to the foregoing, the heat-conductive coated layer 102 is transformed automatically into a hemispherical shape following the shape of the thermistor 1 to be positioned behind the thermistor, and both sides of the thin metal sheet remain at the top surface and come in contact with the fixing roller 10 under the pressure-contact condition. Therefore, the high heat-storing effect can be achieved. Further, since the thermistor 1 is held so that a part thereof is protruded from the top surface of the supporting member 3 by reaction force caused by elasticity of the supporting member 3, the thermistor 1 can be kept in pressure-contact with the surface of fixing roller 10 maintaining the firm contact condition despite dispersion of thermistor 1 itself and erroneous positioning thereof. Figs. 5(a) - 5(d) show constitution of the third example of a temperature sensing device wherein supporting member 12 that supports thermistor 1 is of a two-layer composition including elastic member 12A and elastic member 12B both of which are cemented to be one unit. For the elastic member 12A being in contact with thermistor 1, foamed material having high thermal conductivity is used, and for the elastic member 12B, on the other hand, foamed material being highly adiabatic is used, and they are united into one in a flat layer form as shown in Fig. 5(a) or in an embedded type as shown in Fig. 5(b). Even in the present example, therefore, when being brought into contact with the surface of fixing roller 10, the thermistor 1 is embedded in the elastic member 12A as shown in Fig. 5(c) and Fig. 5(d) showing a sectional view taken on arrowed line BB in Fig. 5(c), and thereby high heat-storing effect is obtained. Fig. 6 shows constitution of the fourth example of a temperature sensing device wherein metallic holder 14 holding the supporting member 12 has recessed portion 14A at the central part on the bottom of the metallic holder, and a slight clearance is formed between the recessed portion and the bottom of the supporting member 12. Due to the foregoing, when the metallic holder 14 is urged clockwise by the action of torsion spring 8 and thereby the supporting member 12 is brought into pressure-contact with the circumferential surface of the fixing roller 10, an upstream side and downstream side of the supporting member 12 in the direction of rotation of the fixing roller, namely edge portions at right and left sides of the supporting member 12 are brought into pressure-contact strongly, while the thermistor 1 is brought into pressure-contact moderately owing to the elasticity of the supporting member 12. Therefore, a stable temperature sensing attitude can be maintained constantly without any change thereof and without any generation of vibrations even when friction resistance caused by sliding of the fixing roller 10 is applied thereon. Still other examples of the invention will be explained as follows, referring to Figs. 7(a) - 7(c) and Figs. 8(a) - 8(c). Figs. 7(a) - 7(c) show basic constitution of the fifth examples of a temperature sensing device wherein the numeral 1 represents a thermistor that is a temperature sensing element, the numeral 3 is an elastic supporting member, 202 represents a highly heat-conductive film-shaped sheet that covers the surface of thermistor 1, and 4 is a metallic holder that holds supporting member 3. For the supporting member 3 mentioned above, foamed silicone material that has broad elastic area and is highly heat-resisting is used, and one side of the supporting member 3 is provided with thermistor 1 and film-shaped sheet 202 which is highly heat-conductive is formed on the surface of a temperature-sensing portion of the thermistor 1 through the evaporating means or the like to cover the thermistor. Incidentally, the film-shaped sheet 202 may also be formed through the means such as glueing, spattering or coating. As is shown in Fig. 7(a), both the thermistor 1 and supporting member 3 covered by highly heat-conductive film-shaped sheet 202 are wrapped with heat-resisting protective sheet 5 such as a polyimide film together with the metallic holder 4 to be united in one. A lead wire of the thermistor 1 is welded with an electric cable through the means such as welding in the supporting member 3 and the electric cable is led out of the supporting member 3. As shown in Fig. 7(b) and Fig. 7(c) that shows a sectional view taken on arrowed line AA in Fig. 7(b), therefore, the thermistor 1 is caused by the tension based on wrapping by means of the protective sheet 5 to be buried partly in the supporting member 3, forming a hemispherical concave on the top face of the supporting member 3. As a result of the constitution mentioned above, when the thermistor 1 is in contact with the fixing roller 10, a temperature sensing portion of the thermistor 1 is totally covered with temperature of the fixing roller 10 that is an item to be measured, thereby it is possible to enhance temperature sensing ability and heat responding ability both for fixing roller 10. Further, since the thermistor 1 is held so that a part thereof is protruded from the top surface of the supporting member 3 by reaction force caused by elasticity of the supporting member 3, the thermistor 1 can be kept in pressure-contact with the surface of fixing roller 10 maintaining the firm contact condition despite dispersion of thermistor 1 itself and erroneous positioning thereof. Figs. 8(a) - 8(c) show the constitution of the sixth examples of a temperature sensing device wherein thermistor 1 is sandwiched vertically between two equally-shaped highly heat-conductive sheets 112 and is placed on supporting member 3 held by metallic holder 4 to be wrapped with protective sheet 5 so that they may be united solidly. With regard to the thermistor 1, the supporting member 3, the metallic holder 4 and the protective sheet 5, those having the same specific characteristics as in the fifth example may be used. As heat-conductive sheet 112 having high thermal conductivity, a metal foil such as an aluminum foil is preferably used. Fig. 8(a) shows how thermistor 1 sandwiched by heat-conductive sheets 112 is placed on supporting member 3, Fig. 8(b) shows how a portion of thermistor 1 wrapped with protective sheet 5 to be united solidly which is protruded in a shape of a semicircle is brought into contact with the circumferential surface of fixing roller 10, and Fig. 8(c) shows a sectional view taken on arrowed line BB in Fig. 8(b). In the present example, again, the thermistor 1 is in the state that it is sandwiched by heat-conductive sheets 112 which represent a heat-absorbing member and thereby, both the top surface and the bottom surface of the thermistor 1 are mostly the same as the circumferential surface of the fixing roller 10 in terms of temperature. Therefore, it has been confirmed that temperature sensing with a small temperature gradient for the thermistor 1 and an excellent responding ability can be conducted. In the example mentioned above, the thermistor 1 is sandwiched between two heat-conductive sheets 112. However, the thermistor 1 may be inserted in twice-folded heat-conductive conductive sheet 112 or may be put in bag-shaped heat-conductive sheet 112 for being placed on the supporting member 3. This may contribute to the easy assembling work. Owing to the present invention, it has become possible to provide a fixing device with a temperature sensing device that is suitable for mass production because of its merits of easy processing and mounting thereof and is capable of measuring and controlling the fixing temperature accurately with its advantages of firm contact with a fixing roller and high heat-storing effect.

Claims: Temperature sensing device for sensing a temperature of a fixing roller (10) in a fixing device for fixing with heat a toner image on a recording material, comprising: a) a temperature sensing element (1); b) a resilient support member (3,12) for supporting the sensing element (1); and c) a protective sheet (5) by which the sensing element (1) and the support member (3,12) are integrally wound, wherein when said sensing device is kept in press contact with the roller (10), one side of the sensing element (1) is urged to sink partly into the support member (3,12) and the opposite side of the sensing element (1) is kept in press contact with the roller (10) at least through the protective sheet (5), characterized by d) a heat conductive layer (2,102,12A,202,112) provided between the sensing element (1) and the support member (3,12) so as to press with the resilience of the support member (3,12), wherein when said sensing device is kept in press contact with the roller (10), a part of the conductive layer (2,102,12A,202,112) is in press contact with the roller (10) through the protective sheet (5). Temperature sensing device according to claim 1, characterized in that said heat conductive layer is a thin metal plate (2). Temperature sensing device according to claim 2, characterized in that said metal plate (2) is one of aluminum foil, copper foil, beryllium foil and magnesium foil. Temperature sensing device according to claim 1, characterized in that said heat conductive layer is a coated layer (102) of heat conductive material coated on the portion where said sensing element (1) is in contact with said support member (3,12). Temperature sensing device according to claim 1, characterized in that said heat conductive layer is an elastic member (12A) including a material having thermal conductivity. Temperature sensing device according to claim 1, characterized in that said heat conductive layer is a film-shaped sheet (202) with which a surface of a heat sensing portion of said sensing element (1) is covered. Temperature sensing device according to claim 6, characterized in that said film-shaped sheet (202) is formed by means such as glueing, spattering or coating. Temperature sensing device according to claim 1, characterized in that said heat conductive layer is a heat conductive sheet (112), and that a part of said sheet (112) is provided between said sensing element (1) and said roller (10). Temperature sensing device according to one of the preceding claims, characterized in that said support member (3) is a foamed silicone. Temperature sensing device according to one of the preceding claims, characterized in that a holder (14) for pressing said support member (12) against said roller (10) exerts the pressing force on two portions of the support member (12) which are located upstream and downstream from the sensing element (1) with respect to the rotational direction of the roller (10).

Inventors: HANADA KOUICHI, NAKAGAMA KIYOHARI, HANADA, KOUICHI, NAKAGAMA, KIYOHARI, Hanada, Kouichi, c/o Konica Corporation, NAKAGAMA, KIYOHARI, C/O KONICA CORPORATION
Applicants: KONISHIROKU PHOTO IND, KONICA CORPORATION
Application year: 1992
Main classification: G01K13
IPC codes: G01K13, G03G15