Patent Publication Number: US-9423738-B1

Title: Heat generating unit, fixing unit, and image forming apparatus having a thermal destruction element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-129471 filed Jun. 29, 2015. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a heat generating unit, a fixing unit, and an image forming apparatus. 
     (ii) Related Art 
     In recent years, to achieve an energy-saving and convenient fixing unit and image forming apparatus that require short rise time, there is a demand for reduction in heat capacity of a heating source, such as a heater, and a member to be heated, such as a fixing belt, of a fixing unit and image forming apparatus in which a fixing belt is heated by a heater (heat generating unit) disposed inside an endless fixing belt, through heat conduction. 
     Such a fixing unit and image forming apparatus having reduced heat capacity tend to cause overheating due to the small heat capacity, so, there is also a demand for a mechanism for preventing fuming and smell due to overheating, occurring when the temperature control becomes defective. 
     SUMMARY 
     According to an aspect of the invention, there is provided a heat generating unit including a substrate; a heat-generating element that is provided on the substrate and generates heat by receiving electric power; and a thermal destruction element provided on the substrate and connected in series to the heat-generating element, the thermal destruction element having a positive temperature coefficient and causing thermal destruction due to self-heating when heated to a temperature higher than a certain temperature by the heat of the heat-generating element. Note that “a certain temperature” as used herein is a temperature at which the maximum resistance can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic diagram showing the configuration of a printer, serving as an exemplary embodiment of an image forming apparatus of the present invention; 
         FIG. 2  is a sectional view of a fixing unit; 
         FIG. 3  schematically shows the structure of a heater; and 
         FIG. 4  is a graph showing PTC characteristics of a PTC element. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a schematic diagram showing the configuration of a printer, serving as an exemplary embodiment of an image forming apparatus of the present invention. 
     A printer  10  shown in  FIG. 1  is a monochrome printer. An image signal representing an image, generated outside the printer  10 , is input to the printer  10  via a signal cable or the like (not shown). The printer  10  includes a controller  11  that controls the operations of the components inside the printer  10 , and the image signal is input to the controller  11 . In the printer  10 , image formation based on the image signal is performed under the control of the controller  11 . 
     Sheet trays  21  are provided at the bottom of the printer  10 . The sheet trays  21  each accommodate a stack of sheets P. The sheet trays  21  are configured such that they may be freely pulled out for supply of sheets P. The sheet trays  21  may accommodate OHP sheets, plastic paper, envelopes, etc., serving as recording media of the present invention, instead of the paper sheets P. Although the operation of the printer  10  will be described with reference to  FIG. 1 , in which sheets P are accommodated, the basic operation is the same even when other recording media are accommodated. 
     A sheet P in one of the sheet trays  21  is sent to standby rollers  24  by a pickup roller  22  and separating rollers  23 . At the standby rollers  24 , the transportation timing of the sheet P is adjusted, and the sheet P is transported further on. 
     The printer  10  includes a cylindrical photoconductor  12  that rotates in a direction indicated by arrow A. A charger  13 , an exposure unit  14 , a developing unit  15 , a transfer unit  16 , and a photoconductor cleaner  17  are arranged around the photoconductor  12 . The photoconductor  12 , the charger  13 , the exposure unit  14 , the developing unit  15 , and the transfer unit  16  are collectively an example of a forming unit of the present invention. 
     The charger  13  charges the surface of the photoconductor  12 , and the exposure unit  14  exposes the surface of the photoconductor  12  according to an image signal transmitted from the controller  11 , thus forming an electrostatic latent image. The electrostatic latent image is developed by the developing unit  15  into a toner image. 
     Herein, the standby rollers  24  send the sheet P such that the sheet P reaches a position facing the transfer unit  16 , at the time when the toner image on the photoconductor  12  reaches the aforementioned position. Then, the toner image on the photoconductor  12  is transferred to the sheet P sent to the aforementioned position by the transfer unit  16 . In this manner, an unfixed toner image is formed on the sheet P. 
     The sheet P having the unfixed toner image thereon moves further in an arrow B direction and is heated and pressed by a fixing unit  18 . Thus, the toner image is fixed onto the sheet P. As a result, an image, formed of a fixed toner image, is formed on the sheet P. The fixing unit  18  corresponds to an exemplary embodiment of a fixing unit of the present invention. 
     The sheet P that has passed through the fixing unit  18  advances in an arrow C direction toward an output unit  19 . The sheet P is further sent in an arrow D direction by the output unit  19  and is output onto a sheet output tray  20 . 
       FIG. 2  is a sectional view of the fixing unit  18 . 
     The fixing unit  18  includes a pressure roller  110  and a heating roller  120 . 
     The pressure roller  110  is formed of a metal core and a rubber layer formed thereon. The pressure roller  110  rotates in an arrow E direction. The pressure roller  110  is an example of a pressure member of the present invention. 
     The heating roller  120  has an outer circumferential belt  121 . A heater  122 , a pressure pad  123 , etc. are accommodated inside the outer circumferential belt  121 . The outer circumferential belt  121  is an example of a revolving member of the present invention, and the heater  122  corresponds to an exemplary embodiment of a heat generating unit of the present invention. 
     The outer circumferential belt  121  of the heating roller  120  revolves in an arrow F direction while being heated by the heater  122  that makes surface contact with the inner circumferential surface of the outer circumferential belt  121 . The outer circumferential belt  121  is urged against the pressure roller  110  by the pressure pad  123 . Thus, force and heat are applied to a sheet P passing between the outer circumferential belt  121  and the pressure roller  110 . 
     The heater  122  has an elongated shape extending in a depth direction of  FIG. 2  and is connected to a power supply at the ends thereof in the longitudinal direction. The heater  122  generates heat by receiving electric power from the power supply. The heater  122  is curved in the direction in which the outer circumferential belt  121  revolves so as to be in contact with the inner circumference of the outer circumferential belt  121 . In order to reduce the rise time, i.e., the time needed for the unheated fixing unit  18  to reach a ready-to-fix state, in this exemplary embodiment, the heater  122  and the outer circumferential belt  121  have small heat capacities. Thus, the heater  122  is configured to suppress overheating when the heat control becomes defective. 
       FIG. 3  schematically shows the structure of a heater  122 . 
     The heater  122  has a structure in which multiple pairs of a heating resistor  132  and a PTC element  133 , connected in series, are arranged side-by-side on a heater base  131 . 
     Although the heater base  131  has an elongated shape extending in the left-right direction in  FIG. 3 , for ease of illustration, the length of the heater base  131  in the longitudinal direction is greatly reduced. The heater base  131  is a plate-shaped member that is curved along the inner circumferential surface of the outer circumferential belt  121 , as shown in  FIG. 2 , and is made of, for example, SUS, copper, clad base material, or the like. The heater base  131  is an example of a substrate of the present invention. 
     The heating resistors  132  are formed of a wiring pattern that is made of, for example, AgPb. Each heating resistor  132  is formed of a wire that forms a series of bends with a width of approximately 15 mm in the longitudinal direction (i.e., the left-right direction in  FIG. 3 ) of the heater base  131  and a length of approximately 20 mm in the transverse direction (i.e., the top-bottom direction in  FIG. 3 ) of the heater base  131 . The heating resistors  132  are an example of a heat-generating element of the present invention. 
     The PTC elements  133  are ceramic elements that are made of, for example, barium titanate mixed with lead. The PTC elements  133  are square flat plates having a thickness of approximately 0.2 mm and a length of each side of approximately 4 mm. The PTC elements  133  are elements having a positive temperature coefficient and are an example of a thermal destruction element of the present invention. 
     Multiple pairs of the heating resistor  132  and the PTC element  133  are arranged side-by-side in the longitudinal direction (i.e., the left-right direction in  FIG. 3 ) of the heater base  131 , and the pairs are connected in parallel by a wire  134 . The wire  134  on the heater  122  is connected to a power supply  140  provided outside the heater  122 , and the heating resistors  132  generate heat using the electric power supplied from the power supply  140 . 
     In this exemplary embodiment, the PTC elements  133  suppress overheating of the heater  122 . A detailed description will be given below. 
       FIG. 4  is a graph showing the PTC characteristics of the PTC elements  133 . 
     In  FIG. 4 , the horizontal axis indicates the temperature, and the vertical axis indicates the resistance. 
     A graph curve  150 , which shows the PTC characteristics of the PTC elements  133  employed in this exemplary embodiment, steeply rises at a temperature exceeding a Curie temperature Tc. This shows that the resistance of the PTC elements  133  steeply increases when the temperature of the elements exceeds the Curie temperature Tc. As a result, the ratio of a minimum resistance Rmin at a temperature lower than the Curie temperature Tc to a maximum resistance Rmax at a temperature higher than or equal to the Curie temperature Tc typically exceeds 1:100, and sometimes it reaches 1:100000. 
     Such ceramic elements are used as the PTC elements  133  shown in  FIG. 3 , and the Curie temperature Tc is adjusted to a temperature higher than a normal use temperature in the heater  122  and lower than an abnormal temperature at which fuming or smell occurs, by adjusting the amount of lead mixed. Furthermore, although the minimum resistance Rmin and the maximum resistance Rmax are determined according to the size of the PTC elements  133 , in this exemplary embodiment, the minimum resistance Rmin is set to less than or equal to one twenty-fifth of the resistance of the heating resistors  132 , so that heat generation by the heating resistors  132  is not affected at the normal use temperature. 
     Because these PTC elements  133  are arranged as shown in  FIG. 3 , when one of the heating resistors  132  generates excessive heat, the temperature of the PTC element  133  connected thereto exceeds the Curie temperature Tc, and as a result, the resistance of that PTC element  133  steeply increases. Such an increase in resistance causes self-heating of the PTC element  133 , leading to thermal destruction of the PTC element  133  due to the thermal shock caused by the self-heating. Because the thermally destructed PTC element  133  breaks the circuit and immediately shuts off the electric power, overheating of the heating resistor  132  connected in series to that PTC element  133  is quickly suppressed. Because this function of the PTC elements  133  is achieved by the multiple pairs of the heating resistor  132  and the PTC element  133  arranged side-by-side in the longitudinal direction (i.e., the left-right direction in  FIG. 3 ) of the heater base  131 , local overheating of the heater  122  is also suppressed. 
     As has been described above, the PTC elements  133  have a flat plate shape, which efficiently causes thermal destruction. A critical temperature difference ΔTc that determines whether or not an infinitely spread flat plate is fractured by thermal shock is calculated from the following expression, on the basis of Young&#39;s modulus E, coefficient of linear expansion α, Poisson&#39;s ratio ν, fracture strength σmax, coefficient of heat transfer αM, characteristic length D, and thermal conductivity λ. 
     
       
         
           
             
               
                 
                   
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     When a Young&#39;s modulus E of 1.15×1011 [N/m], a coefficient of linear expansion α of 12.5×1011 [K−1], a Poisson&#39;s ratio ν of 0.3, a fracture strength σmax of 70 [N/m2], a coefficient of heat transfer αM of 1×106 [W/m2K], a characteristic length D of 0.2 [mm], and a thermal conductivity λ of 6 [W/mK], serving as the values of the physical properties, are assigned to the above expression, the resulting critical temperature difference ΔTc is approximately 50K. The above-described maximum resistance Rmax is determined by conducting heat simulation or the like such that self-heating that generates an inside temperature difference of approximately 50K or more occurs, and, according to the thus-determined maximum resistance Rmax, the size of the PTC elements  133  is determined. By determining the maximum resistance Rmax in this way, thermal destruction of the PTC elements  133  is reliably caused, making it possible to reliably suppress overheating of the heating resistors  132 . Furthermore, because the thus-determined maximum resistance Rmax is the resistance for causing self-heating, it is much smaller than the resistance for suppressing the current flow by increasing the resistance. Hence, the size and heat capacity of the PTC elements  133  are reduced, enabling thermal destruction to be caused immediately in response to overheating of the heating resistors  132 . 
     In the above-described exemplary embodiment, although a ceramic element composed in large part of barium titanate has been shown as an example thermal destruction element of the present invention, the thermal destruction element of the present invention may be a ceramic element that is composed in large part of a material other than barium titanate or a non-ceramic element, as long as it causes thermal destruction. 
     Furthermore, in the above-described exemplary embodiment, the curved heater  122  that comes into contact with the inner circumference of the outer circumferential belt  121  has been shown as an exemplary embodiment of the heat generating unit of the present invention, the heat-generating member of the present invention may be one that has a flat-plate shape, one that comes into contact with the outer circumferential of the outer circumferential belt  121  for heating, one that heats a metal tube or the like other than the outer circumferential belt  121 , or one that is used for heating in a unit other than the fixing unit  18 . 
     Furthermore, although a monochrome printer has been shown as an example in the above-described exemplary embodiment, the present invention may be applied to a color printer, or it may be applied to a facsimile, a copier, or a multi-function apparatus. 
     Furthermore, although a device for forming a toner image using an electrophotographic system has been shown as an example in the above-described exemplary embodiment, the forming unit of the present invention may be one that forms a toner image on a recording medium by using a method other than the electrophotographic system. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.