Patent Publication Number: US-6661992-B2

Title: Fusing roller for an electrophotographic image forming apparatus having quick warm up time and uniform temperature distribution

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application entitled FUSING ROLLER OF ELECTROPHOTOGRAPHIC IMAGE FORMING APPARATUS filed with the Korean Industrial Property Office on Nov. 16, 2001 and there duly assigned Serial No. 2001-0071399. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fusing roller in an electrophotographic image forming apparatus, and more particularly, to a design for a fusing roller used in an electrophotographic image forming apparatus that minimizes a surface temperature gradients over the surface of the fusing roller. 
     2. Description of the Related Art 
     Designs of fusing roller units adopting a heat pipe capable of instantaneously heating the fusing roller due to high heat conductivity an having low power consumption are disclosed in Japanese Patent Publication Nos. Hei 5-135656, Hei 10-84137, Hei 6-29663, and Hei 10-208635. Such fusing roller units adopting the heat pipe have a structure in which heat sources having different shapes are provided at one side end of the fusing roller unit deviating from a fusing region. In the arrangement structure of heat sources, the overall length of the fusing roller unit may be enlarged, and thus structural complication should be improved. 
     Also, the fusing roller units disclosed in Japanese Patent Publication Nos. Sho 58-163836, Hei 3-107438, Hei 3-136478, Hei 6-316435, Hei 7-65878, and Hei 7-105780, and Hei 7-244029 have a structure in which heat sources are provided inside the fusing rollers, and thus the above-mentioned enlargement of the overall length does not occur. However, the fusing roller units have a plurality of partial heat pipes, and thus processing and manufacturing of the fusing roller units are very complicated. 
     Further, the heat pipes are arranged partially in the fusing roller units, and thus a temperature difference between a portion among the heat pipes and a portion contacting the heat pipes occurs. When a temperature difference occurs in the fusing roller, ink such as toner, is not properly transferred onto the paper, degrading printing quality. 
     Serial Nos. 60/257,118, 09/947,657 and 09/967,934 teach a fusing roller that has a resistive heating element wound about the fusing roller in a spiral fashion. Near the axis of the fusing roller is a heat pipe comprising a working fluid and a wick. Although this design of a fusing roller enables the fusing roller to be heated quickly, the surface of the fusing roller has an unwanted temperature gradient along the length of the fusing roller. When recently heated, the center portion of the fusing roller is at a higher temperature than the surface of the fusing roller near the ends of the fusing roller. 
     What is needed is a design for a fusing roller that eliminates these temporary temperature gradients along the length of the surface of the fusing roller by compensating for the temperature gradients along the length of the surface of the fusing roller. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a design for a fusing roller used in an electrophotographic image forming apparatus, the fusing roller including a heating portion having an improved structure in which the above-referenced surface temperature gradients of the fusing roller can be minimized, and thus a high quality fusing of the toner image can be achieved with minimal warm up times. 
     To achieve the above object, according to one aspect of the present invention, there is provided a novel design for a fusing roller used in an electrophotographic image forming apparatus. The fusing roller includes an internal pipe having enclosed both ends in which a vacuum state of predetermined pressure is maintained and a predetermined amount of working fluid is stored, a fusing portion installed to surround the internal pipe, and a heating portion comprised of a resistance heating coil which is installed between the fusing portion and the internal pipe and generates heat. In order to compensate for the roll off in temperatures near the ends of the fusing roller, the resistive heating element near the end portions of the fusing roller are spaced closer together than at the center of the fusing roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a latitudinal cross-sectional view of a fusing roller unit of an electrophotographic image forming apparatus in which a halogen lamp is used as a heat source; 
     FIG. 2 is a longitudinal cross-sectional view illustrating the relationship between the fusing roller unit and a pressure roller of an electrophotographic image forming apparatus in which the halogen lamp shown in FIG. 1 is used as a heat source; 
     FIG. 3 is a longitudinal cross-sectional view of a fusing unit according to the present invention; 
     FIG. 4 is a latitudinal cross-sectional view illustrating the structure of a fusing roller shown in FIG. 3; 
     FIG. 5 is an enlarged view of a center portion C illustrated in FIG. 4; 
     FIG. 6 is an enlarged view of an end portion E illustrated in FIG. 4; 
     FIG. 7 is a graph illustrating empirical data of the temperature with respect to time of the surface of the fusing roller both near the ends and near the center of the fusing roller when the distance between adjacent winds of the heater resistive element is uniform and constant along the entire length of the fusing roller; and 
     FIG. 8 is a graph illustrating empirical data of the temperature with respect to time of the surface of the fusing roller both near the ends and near the center of the fusing roller when the distance between adjacent winds of the heater resistive element is smaller near the ends of the heating roller than at the center of the heating roller. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a latitudinal cross-sectional view of a fusing roller unit used in an electrophotographic image forming apparatus where a halogen lamp is used as the heat source. Referring to FIG. 1, a fusing roller unit  10  includes a cylindrical fusing roller  11  having a heating portion  12 , such as halogen lamp installed along the axis of the fusing roller  11 . A coating layer  11   a  formed of Teflon or the like is formed on the surface of the fusing roller  11 . The heating portion  12  generates heat from the inside of the fusing roller  11 , and the fusing roller  11  is heated by the radiant heat generated by the heating portion  12  from the inside of the fusing roller  11 . 
     FIG. 2 is a longitudinal cross-sectional view illustrating the relationship between the fusing roller unit  10  and a pressure roller  13  in an electrophotographic image forming apparatus where the halogen lamp illustrated in FIG. 1 is used as a heat source. Referring to FIG. 2, pressure roller  13  is disposed under the fusing roller unit  10  to be opposite to the fusing roller unit  10  in which a paper  14  is placed between the fusing roller unit  10  and the pressure roller  13 . The pressure roller  13  is supported elastically by a spring unit  13   a  and applies predetermined pressure to the paper  14  passing between the fusing roller unit  10  and the pressure roller  13 . 
     In FIG. 2, a powdered toner image  14   a  is formed on the paper  14 , and the paper  14  is pressed and heated by the predetermined pressure and is heated while passing between the fusing roller unit  10  and the pressure roller  13 . This heat and pressure causes the toner image  14   a  to be fused on the paper  14 . 
     One drawback of using a halogen lamp as a heating source is that there is much unnecessary power consumption by the fusing roller unit  10  in which a halogen lamp is used as a heat source. This is manifested by longer warm-up times after the power is turned off and is turned back on again to form images. In addition, since the fusing roller  11  is heated by radiant heat generated by the heating portion  12  installed along the axis of the fusing roller  11 , the heat transfer speed of the fusing roller unit  10  using this halogen lamp is slow. Also, the compensation of temperature changes caused by contact with the paper  14  is slow, making it difficult to control the distribution of temperature on the surface of the fusing roller thereby degrading the quality of an image. In addition, an image cannot be printed quickly after the printer has been powered down because of the long warm up time. 
     FIG. 3 is a longitudinal cross-sectional view of a fusing unit according to the present invention, and FIG. 4 is a latitudinal cross-sectional view illustrating the structure of a fusing roller illustrated in FIG.  3 . Referring to FIGS. 3 and 4, a fusing unit  200  used in an electrophotographic image forming apparatus according to the present invention includes a fusing roller  210  which rotates in a direction in which a paper  250  on which a toner image  251  is formed is conveyed. Thus, fusing roller  210  rotates in the direction indicated by arrow A and pressure roller  500  rotates in a direction indicated by arrow B so that fusing roller  210  and pressure roller  500  form a nip that conveys paper  250  from right to left in FIG.  3 . 
     The fusing roller  210  includes a cylindrical fusing portion  212  in which a protection layer  211  formed by coating Teflon on its surface is formed, a heating portion  213  which is installed inside the fusing portion  212  and generates heat due to current applied from a power supply portion  300 , and an internal pipe  214  having an enclosed internal space, in which predetermined pressure is maintained. A first insulating layer  216   a  is interposed between the heating portion  213  and the internal pipe  214 , and a second insulating layer  216   b  is interposed between the heating portion  213  and the fusing portion  212 . 
     In an embodiment of the present invention, preferably, the first insulating layer  216   a  and the second insulating layer  216   b  are mica. The first insulating layer  216   a  and the second insulating layer  216   b  are optional and may be not formed. Preferably, a net-shaped wick structure is provided inside the internal pipe  214  to enable the working fluid  215  inside pipe  214  to evaporate and condense as described in Serial Nos. 60/257,118, 09/947,657 and 09/967,934 to more evenly heat the surface  211  of the fusing roller. Of course, various modifications in which heat can be uniformly transferred to the entire portion of the internal pipe  214  may be possible. 
     Working fluid  215  is stored in the internal pipe  214  at a predetermined volume ratio. Also, the power supply portion  300  in frame  400 , power supply portion being connected to an external power supply (not shown) for supplying current to the heating portion  213 . A thermistor  230  which contacts the protection layer  211  and senses the surface temperature of the fusing portion  212  and the protection layer  211 , and thermostat  240  which prevents overheat of the fusing portion  212  by cutting power from the power supply portion  300  when the surface temperature of the fusing portion  212  and the protection layer  211  rapidly increases, are installed on the fusing portion  212 . 
     An end cap  218  for enclosing inside the fusing roller  210  and a gear cap  219  having an additional power transmission device (not shown) are provided at both ends of the fusing roller  210 . Thus, the fusing roller  210  rotates by the gear cap  219  connected to the power transmission device. Preferably, the gear cap  219  and the power transmission device are constituted by smoothly-engaged gears. 
     An electrode  220  for applying current supplied from an external power to the heating portion  213  is installed in the end cap  218 . One side of the electrode  220  is electrically connected to the heating portion  213  by a lead portion  213   a , and the other side of the electrode  220  is electrically connected to the power supply portion  300 . Since the power supply portion  300  adheres to the electrode  220  through an elastic member  310 , even through the fusing portion  212  rotates, the electrode  220  continuously contacts the power supply portion  300 , and the current supplied from the power supply portion  300  is applied to the heating portion  213  through the electrode  220  and the lead portion  213   a.    
     The heating portion  213  generates heat such that the temperature of the fusing portion  212  increases to a fusing target temperature so as to fuse the toner image  251  on the paper  250  by the current supplied from an external power source, heating portion  213  being preferably formed of a resistance heating coil. The resistance heating coil has a helical shape or a ribbon shape and may be formed of molybdenum (Mo) or tungsten (W), or alloy of iron-chrome (Fe—Cr), or alloy of nickel-chrome (Ni—Cr), or alloy of copper-nickel (Cu—Ni). The resistance of the resistance heating coil is 4-20 Ω/m, and its volume resistivity is 0.4-1.55 μΩ·m. The resistance heating coil is installed to be wound around the internal pipe  214  in a spiral such that a predetermined separated space D is formed between adjacent heating coils. 
     FIG. 5 is an enlarged view of a center portion C illustrated in FIG. 4, and FIG. 6 is an enlarged view of an end portion E illustrated in FIG.  4 . In both FIGS. 5 and 6, resistance heating coil  213  has a width W and a thickness T at all locations on the fusing roller. In other words, in this embodiment of the present invention, the width W of resistance heating coil  213  is constant and the thickness T is constant. 
     In this embodiment of the present invention, the distance D separating adjacent windings varies near each end of the fusing roller. Referring to FIG. 5, the distance D 1  between adjacent windings is constant throughout section C of FIG.  4 . However, in FIG. 6, the distance D between adjacent windings progressively decreases in section E of FIG.  4 . In FIG. 6, D 4  is less than D 3  as D 4  is closer to an end of the fusing roller than D 3 . Since the width W of resistive heating coil  213  in this embodiment is always constant, changes to the distance D between adjacent coils results in changes in pitch between adjacent winding as the pitch is equal to the sum of the width W and the distance between a pair of adjacent windings D. Thus, in this embodiment, both the distance D between adjacent windings and the pitch of each winding progressively gets smaller towards the ends of the roller while the thickness, composition and width of the heating coil remains the same. 
     FIG. 7 is a graph illustrating temperature with respect to time at different points on the fusing roller when the distance D between adjacent coils of the resistive heating element  213  is constant and thus the pitch is constant from end to end on the fusing roller. Referring to FIG. 7, if the temperature of the fusing portion  212  is empirically measured when the distance between adjacent coils and pitch are constant from end to end, the surface temperature at the end portions of the fusing roller are initially cooler than the surface temperature at the center portion of the fusing roller. This is because the amount of heat dissipated at the end portions of the fusing roller  210  is greater than the amount of heat dissipated from the center portion of the fusing roller. Thus, even though the heat produced by resistive heating element  213  along the axis of the fusing roller is uniform, the temperature measured at both ends of the fusing roller  210  is lower than the temperature measured at the center portion of the fusing roller  210 . 
     FIG. 8 is a graph illustrating temperature with respect to time at different points on the fusing roller when the distances between adjacent coil windings and the pitches become progressively smaller towards the ends of the fusing roller. Referring to FIG. 8, if the temperature of the fusing portion  212  is measured when the distances between adjacent coils of a resistance heating element  213  become progressively smaller at the ends of the fusing roller, the empirically measured temperature at each end portion of the fusing roller  210  is about the same as the temperature measured at the center portion of the fusing roller  210 . By spacing the coils closer to one another at the end portions of the fusing roller, more heat is generated at the end portions than at the center of the fusing roller. Since more heat is also dissipated at the end portions of the fusing roller, this increased generation of more heat at the end portions of the fusing roller balances out the increase in dissipation of heat at the end portions, resulting in the temperature at the end portions of the fusing roller to be equal to the temperature at the center portion of the fusing roller. Since the temperature is now uniform along the entire length of the fusing roller, better quality printing is achieved while benefitting from a quick warm up time of the fusing roller. 
     As described above, in the fusing roller used in an electrophotographic image forming apparatus according to the present invention, the distance between coils become progressively smaller at the ends of the fusing roller, thereby resulting in a uniform temperature along the entire length of the fusing roller at all times after power is applied to the fusing roller. Therefore, high image quality and quick warm up times can now be achieved by using the above design for a fusing roller. 
     Although the present invention seeks to compensate for increased heat dissipation at the ends of the fusing roller by solely adjusting the space between adjacent coils of a resistive heating element at the ends of a fusing roller, it is to be appreciated that this invention is not limited solely to this form of heat compensation. It is to be appreciated that the heating resistance coil  213  can be made thinner and/or narrower at the ends of the fusing roller than at the center of the fusing roller while keeping the pitch between adjacent coils the same throughout the length of the fusing roller. Alternatively, the heating resistance coil can be made of a material having a higher resistivity at the end portions of the fusing roller than at the center of the fusing roller. In addition, it can be appreciated that a combination of 1) decreasing the space between adjacent coils, 2) decreasing the width of the heating resistive element, 3) reducing the thickness of the heater resistive element at the ends of the fusing roller and/or 4) using a different and higher resistivity material as the heater resistive element at the ends of the fusing roller than used in the center of the fusing roller can all be applied in combination to compensate for the increased amount of heat dissipation at the ends of the fusing roller than at the center of the fusing roller. A further embodiment is contemplated where the pitch of the coil and the distance between windings is smaller at the ends of the roller than at the center, where the pitch and the distance between windings of the coil at the end portions are constant as opposed to becoming progressively smaller. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.