Patent Publication Number: US-6665515-B2

Title: Fusing device for electrophotographic image forming apparatus

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 FUSING DEVICE OF ELECTROPHOTOGRAPHIC IMAGE FORMING APPARATUS filed with the Korean Intellectual Property Office on Aug. 25, 2001 and there duly assigned Serial No. 51583/2001, which was subsequently published on the Mar. 4, 2003 as Publication No. 2003-17940 by the Korean Intellectual Property Office. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a fusing device for an electrophotographic image forming apparatus and, more particularly, to a fusing device using a heat pipe to decrease power consumption and allow flash heating in an electrophotographic image forming apparatus. 
     2. Related Art 
     Electrophotographic image forming apparatuses include a fusing device for heating a sheet, to which a toner image is transferred, to fuse and fix the toner image in a powder state to the sheet. The fusing device includes a fusing roller for fusing and fixing a toner to a sheet and a pressing roller for pressing the sheet against the fusing roller. 
     A fusing roller unit includes a cylindrical fusing roller and a halogen lamp installed inside the fusing roller and along its axis. A Teflon coating layer is formed on the surface of the fusing roller. The halogen lamp generates heat within the fusing roller, and the fusing roller is heated by the radiant heat emitted from the halogen lamp. 
     A pressing roller is disposed below the fusing roller unit and in contact with the fusing roller such that a sheet passes therebetween. The pressing roller is elastically supported by a spring so that it makes the sheet closely contact the fusing roller with a predetermined pressure when the sheet passes between the fusing roller and the pressing roller. A toner image formed on the sheet in a powder state is fused and fixed to the sheet by predetermined pressure and heat when the sheet passes between the fusing roller and the pressing roller. 
     A thermistor for measuring the surface temperature of the fusing roller and a thermostat for cutting off the supply of power when the surface temperature of the fusing roller exceeds a predetermined set value are provided at one side of the fusing roller. The thermistor measures the surface temperature of the fusing roller and transmits an electric signal corresponding to the measured temperature to a controller of a printer. The controller controls the quantity of electricity supplied to the halogen lamp according to the measured temperature so as to maintain the surface temperature of the fusing roller within a predetermined range. When the temperature of the fusing roller exceeds the predetermined set value because the thermistor and the controller fail to control the temperature of the fusing roller, a contact of the thermostat opens so as to cut off the supply of power to the halogen lamp. 
     Such a fusing device using a halogen lamp as a heat source consumes a large amount of electric power. Particularly, when power is turned on, the device requires quite a long warming-up time. The warming-up time may range from several tens of seconds to several minutes. In addition, in such a fusing device, since the fusing roller is heated by radiation emitted from a heat source, heat transmission is slow, and compensation for temperature deviation caused by a decrease in temperature occurring due to contact with a sheet is slow. Thus, it is difficult to maintain the temperature of the fusing roller constant. Moreover, since electric power must be periodically applied to the heat source in order to maintain the temperature of the fusing roller constant in a standby mode in which the operation of the printer is in a pause state, unnecessary electric power is consumed. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is an object of the present invention to provide a fusing device which includes a power connecting unit which has improved durability and reliability and which decreases a warming-up time at initial operation or at transition from a standby mode to re-operation in an electrophotographic image forming apparatus. 
     To achieve the above object of the invention, there is provided a fusing device for an electrophotographic image forming apparatus. The fusing device includes: a heat pipe having a tubular shape and containing a predetermined amount of working fluid, the heat pipe being hermetically sealed at both of its ends; a fusing roller surrounding the heat pipe; a heater installed between the fusing roller and the heat pipe for generating heat; and a power connecting unit for transmitting external electric power to the heater. The heater includes: a resistive coil for generating heat using the electric power transmitted by the power connecting unit, the resistive coil not being covered with a protective coating layer; a first insulation layer provided on the inside of the fusing roller so as to contact the resistive coil; a second insulation layer provided on the outside of the heat pipe so as to contact the resistive coil; and leads for connecting the resistive coil to the power connecting unit at both ends of the heater. 
     Preferably, each of the first and second insulation layers is formed of at least one mica layer. 
     Preferably, the power connecting unit includes an electrode inserted into an outer end portion of each of first and second end caps which are installed at both ends of, and on the axis of rotation of, the fusing roller, a brush installed in a through hole formed in a frame supporting the fusing roller so as to contact the electrode, and an elastic unit for making the brush closely contact the electrode for electrical connection. 
     Preferably, each of the first and second end caps includes a lead hole formed in a lengthwise direction so as to allow each lead to pass therethrough, a bottom portion formed so as to allow the lead passing through the lead hole to be electrically connected to the electrode which is inserted into the corresponding end cap, a first insulation film provided on the bottom portion of each of the first and second end caps so as to isolate the bottom portion from the lead, and a heat sink provided on the first insulation film and electrically connected to the lead. 
     Preferably, the fusing device further includes a second insulation film formed in the lead hole of each of the first and second end caps for isolating the lead from the lead hole. 
     Preferably, the fusing device further includes: at least one key formed at a portion of an outer circumference of each end cap, the latter portion engaging an end of the fusing roller; and at least one key way formed at each end of the fusing roller so as to correspond to the key. The key way(s) is (are) formed at the inner side of each end of the fusing roller to be recessed. 
     Preferably, each of the leads includes a ring electrically connected to the resistive coil at each end of the heater, and a string extending from the ring. 
    
    
     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 numerals indicate the same or similar components, and wherein: 
     FIG. 1 is a schematic horizontal sectional view of a fusing roller unit using a halogen lamp as a heat source; 
     FIG. 2 is a schematic vertical sectional view of a fusing device using the fusing roller unit of FIG. 1; 
     FIG. 3 is a schematic vertical sectional view of a fusing device according to a preferred embodiment of the present invention; 
     FIG. 4 is a schematic horizontal sectional view of the fusing roller shown in FIG. 3; 
     FIGS. 5A and 5B are perspective views of a first end cap shown in FIG. 4; 
     FIGS. 6A and 6B are perspective views of a second end cap shown in FIG. 4; 
     FIG. 7 is a sectional view of the first end cap of FIG. 5A, taken along the line VII-VIIN; 
     FIG. 8 is a partial perspective view of the fusing roller of FIG. 4, and shows key ways at an end of the fusing roller; 
     FIG. 9 is an exploded perspective view of a power connecting unit of the fusing roller of FIG. 4; and 
     FIG. 10 is a perspective view of an example of a lead shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the attached drawings. In the drawings, the thickness of films or regions are exaggerated for clarity. 
     FIG. 1 is a schematic horizontal sectional view of a fusing roller unit using a halogen lamp as a heat source, while FIG. 2 is a schematic vertical sectional view of a fusing device using the fusing roller unit of FIG.  1 . 
     Referring to FIG. 1, a fusing roller unit  10  includes a cylindrical fusing roller  11  and a halogen lamp  12  installed inside the fusing roller  11  and along its axis. A Teflon coating layer  11   a  is formed on the surface of the fusing roller  11 . The halogen lamp  12  generates heat within the fusing roller  11 , and the fusing roller  11  is heated by the radiant heat emitted by the halogen lamp  12 . 
     Referring to FIG. 2, a pressing roller  13  is disposed below the fusing roller unit  10  and in contact with the fusing roller  11  such that a sheet  14  passes therebetween. The pressing roller  13  is elastically supported by a spring  13   a  so that it makes the sheet  14  closely contact the fusing roller  11  with a predetermined pressure when the sheet  14  passes between the fusing roller  11  and the pressing roller  13 . A toner image  14   a  formed on the sheet  14  in a powder state is fused and fixed to the sheet  14  by predetermined pressure and heat when the sheet  14  passes between the fusing roller  11  and the pressing roller  13 . 
     A thermistor  15  for measuring the surface temperature of the fusing roller  11  and a thermostat  16  for cutting off the supply of power when the surface temperature of the fusing roller  11  exceeds a predetermined set value are provided at one side of the fusing roller  11 . The thermistor  15  measures the surface temperature of the fusing roller  11  and transmits an electric signal corresponding to the measured temperature to a controller (not shown) of a printer (not shown). The controller controls the quantity of electricity supplied to the halogen lamp  12  according to the measured temperature so as to maintain the surface temperature of the fusing roller  11  within a predetermined range. When the temperature of the fusing roller  11  exceeds the predetermined set value because the thermistor  15  and the controller fail to control the temperature of the fusing roller  11 , a contact (not shown) of the thermostat  16  opens so as to cut off the supply of power to the halogen lamp  12 . 
     Such a fusing device using halogen lamp  12  as a heat source consumes a large amount of electric power. Particularly, when power is turned on, the device requires quite a long warming-up time. The warming-up time may range from several tens of seconds to several minutes. In addition, in the fusing device, since the fusing roller  11  is heated by radiation emitted from a heat source, heat transmission is slow, and compensation for a temperature deviation caused by a decrease in temperature occurring due to contact with a sheet is slow. Thus, it is difficult to maintain the temperature of the fusing roller  11  constant. Moreover, since electric power must be periodically applied to the heat source in order to maintain the temperature of the fusing roller  11  constant in a standby mode in which the operation of the printer is in a pause state, unnecessary electric power is consumed. 
     FIG. 3 is a schematic vertical sectional view of a fusing device, according to an embodiment of the present invention, in an electrophotographic image forming apparatus, while FIG. 4 is a schematic horizontal sectional view of the fusing roller shown in FIG.  3 . Referring to FIGS. 3 and 4, a fusing device for an electrophotographic image forming apparatus according to the present invention includes a fusing roller unit  110  including a fusing roller  112  which rotates in a direction in which a sheet  150  is discharged, that is, clockwise, and a pressing roller  190  which rotates counterclockwise in contact with the fusing roller  112  such that the sheet  150  passes therebetween. 
     The fusing roller unit  110  also includes: cylindrical fusing roller  112 , on the surface of which a coating layer  111  of Teflon coating is formed; a heater  113  which is installed within the fusing roller  112 , and which is supplied with electric power from an external power supply through a power connecting unit  200 ; and a heat pipe  114  which is installed within the heater  113 , and both ends of which are sealed hermetically to maintain a predetermined pressure. The heat pipe  114  accommodates a predetermined volume of working fluid  115 . The power connecting unit  200  is installed at each end of the fusing roller  112 . The power connecting unit  200  is connected to the external power supply so as to transmit electric power to the heater  113 . 
     A thermistor  118  is installed above the fusing roller  112  so that it measures the surface temperature of the fusing roller  112  and the coating layer  111  in contact with the coating layer  111 . Also, a thermostat  119  is installed above the fusing roller  112  so that it cuts off the supply of power in order to prevent overheating when the surface temperature of the fusing roller  112  and the coating layer  111  rapidly increases. 
     The heater  113  includes a resistive coil  113   b  formed of Ni—Cr for generating heat using electric power supplied by the power connecting unit  200 , a first mica layer  113   a  disposed between the resistive coil  113   b  and the fusing roller  112 , a second mica layer  113   c  disposed between the resistive coil  113   b  and the heat pipe  114 , and leads  116  extending outward from both ends of the resistive coil  113   b  so as to be electrically connected to the power connecting unit  200 . Each of the mica layers  113   a  and  113   c  of the heater  113  is composed of at least one layer. The resistive coil  113   b  may be formed of Cr—Fe. 
     In manufacturing the fusing roller unit  110  having the above structure, the heat pipe  114  is sequentially wrapped with the second mica layer  113   c , the resistive coil  113   b , and the first mica layer  113   a , and is then inserted into the fusing roller  112 . Next, a pressure of 100-150 atm. is applied within the heat pipe  114  to enlarge the heat pipe  114  so that the heater  113  can closely contact the outer circumferential surface of the heat pipe  114  and the inner circumferential surface of the fusing roller  112 . 
     The heat pipe  114  has a tubular shape and is hermetically sealed at both of its ends. A predetermined amount of the working fluid  115  is contained in the heat pipe  114 . The working fluid  115  evaporates due to heat generated and transmitted from the heater  113 , and transmits the heat to the fusing roller  112 , thereby functioning as a thermal medium which prevents a difference in the surface temperature of the fusing roller  112  and which heats the entire fusing roller  112  within a short time. The working fluid  115  occupies 5-50% of the interior volume of the heat pipe  114 , and preferably 5-15% of the interior volume of the heat pipe  114 . When the working fluid  115  occupies 5% or less of the interior volume of the heat pipe  114 , a dry-out phenomenon is very likely to occur. Accordingly, it is preferable to avoid the above case of 5% or less. 
     The working fluid  115  is selected depending upon the material of the heat pipe  114 . For example, when the heat pipe  114  is formed of stainless steel, most working fluids known up to now, except for water, can be used as the working fluid  115 . It is most preferable to use FC-40 (3M) as the working fluid  115 . 
     When the heat pipe  114  is formed of copper (Cu), most known working fluids can be used. It is most preferable to use water, i.e., distilled water. Using water or distilled water as the working fluid  115  has the advantages of low cost and prevention of environmental pollution. 
     The fusing roller  112  is heated by heat generated and transmitted by the heater  113 , or by the heat of vaporization of the working fluid  115  contained in the heat pipe  114 , fuses a powder-state toner  151  on the sheet  150 , and fixes the toner  151  to the sheet  150 . The fusing roller  112  is formed of stainless steel, aluminum (Al), or copper (Cu). 
     A first end cap  120  and a second end cap  130  are provided at respective ends of the fusing roller  112  so that both ends of the fusing roller  112  are covered by the first and second end caps  120  and  130 , respectively. The second end cap  130  has the same structure as the first end cap  120 , with the exception that the second end cap  130  is provided with a gear  131  on its outer circumferential surface such that the gear  131  of the second end cap  130  can engage a gear (not shown) of an electric motor so as to cause the second end cap  130  to rotate. 
     FIGS. 5A and 5B are perspective views of the first end cap  120  shown in FIG. 4; FIGS. 6A and  6 B are perspective views of the second end cap  130  shown in FIG. 4; FIG. 7 is a sectional view of the first end cap  120  of FIG. 5A, taken along the line VII-VIIN (a lead  116  is illustrated together for clarity); and FIG. 8 is a partial perspective view of the fusing roller  112  of FIG. 4, and shows key ways at an end of the fusing roller  112 . 
     Referring to FIGS. 5A thru  8 , lead holes  122  and  132  are formed in the first and second end caps  120  and  130 , respectively, so that a lead  116  of FIG. 7 can be introduced into each of the first and second end caps  120  and  130  in a lengthwise direction. Keys  124  and  134  are formed so as to protrude from the inner circumferences of the first and second end caps  120  and  130 , respectively. The keys  124  and  134  engage key ways  112   a  of FIG. 8 formed on the inside surface of both ends of the fusing roller  112 . Recesses  125  and  135  are formed at the centers of the first and second end caps  120  and  130 , respectively, facing both ends of the heat pipe  114  such that both ends of the heat pipe  114  can be inserted into the recesses  125  and  135 . Electrode ways  126  and  136  and electrode receiving portions  127  and  137  are formed in the outer centers opposite to the recesses  125  and  135  of the first and second end caps  120  and  130 , respectively, so as to allow an electrode  210  of FIG. 4 to be inserted into each of the first and second end caps  120  and  130 , respectively. The electrode way  126  is provided, on its bottom  126   a , with a first insulation film  126   b  for preventing the heat of the lead  116  from being conducted to the first end cap  120  and a heat sink  126   c  formed on the first insulation film  126   b  so as to be connected to the lead  116 , thereby radiating the heat of the lead  116 . It is preferable to provide a second insulation film (not shown) on the inside of the lead hole  122  to protect the first end cap  120  from the heat of the lead  116 . 
     FIG. 9 is an exploded perspective view of the power connecting unit  200  connected to the second end cap  130 . Referring to FIG. 9, the power connecting unit  200  is installed within a frame  160  of FIG. 4 so as to transmit external electric power to the heater  113 . The power connecting unit  200  includes an electrode  210  inserted into the electrode way  136  of FIG.  6 A and the electrode receiving portion  137  of FIG. 6A, a brush  220  installed so as to contact the electrode  210  in a through hole formed in the corresponding frame  160  supporting the fusing roller  112  of FIG. 4, and an elastic unit  240  to allow the brush  220  to closely contact the electrode  210  so as to be electrically connected thereto. 
     The electrode  210  includes: a protrusion  212  which is inserted into the electrode way  136  located at the center of the second end cap  130 , i.e., on the axis of rotation of the fusing device  110 ; and a flange  214  integrated with the protrusion  212  and inserted into the electrode receiving portion  137 . The protrusion  212  of the electrode  210  is inserted into the electrode way  136  such that the lead  116 , which is inserted into the lead hole  122  of FIG.  7  and bent at a right angle, can closely contact the bottom of the electrode way  136 , so that the protrusion  212  can be electrically connected to the lead  116 . 
     The first and second end caps  120  and  130 , respectively, can be formed of polyphenylene sulfide (PPS), polybutylene terephthalate (PBT), or nylon, and has a filler such as glass fiber which transforms only slightly, even at high temperature. 
     The brush  220  is connected to the electrode  210  so as to transmit external electric power, and is composed of a projection  222  and a plate  224 . The projection  222  contacts the flange  214 , and the plate  224  is connected to external lead  254  of FIG.  4 . 
     A through hole is formed in the frame  160 . A first stopper  162  and a second stopper  164  are sequentially formed in the through hole starting from its side nearer to the fusing roller  112 . When the brush  220  is inserted into the through hole, the first stopper  162  stops and supports the plate  224 . The second stopper  164  stops and supports a flange  251  of an insulation plate  250 . 
     The elastic unit  240  gives elasticity to a spacer  130  so that the brush  220  closely contacts the electrode  210 . In addition, the elastic unit  240  buffers transformation due to thermal expansion or contraction during repeated operation of the fusing roller  112 , thereby preventing the brush  220  from being disconnected from the electrode  210 . Accordingly, it is preferable to use a compression spring as the elastic unit  240 . The external lead  254  of FIG. 4 is connected to the brush  220  through a lead hole  252 . The lead  254  may dangerously contact the elastic unit  240 , provoking a spark. Accordingly, in order to prevent this danger from occurring, and to prevent the second end cap  130  from contacting the frame  160  due to a withdrawal of the brush  220 , a spacer  230  is provided. 
     The elastic unit  240  is installed in the frame  160  by using the insulation plate  250 . The insulation plate  250  supports the elastic unit  240 . Accordingly, the brush  220  is installed in the through hole of the frame  160  first, and then the elastic unit  240  and the spacer  230  are installed. Next, the insulation plate  250  is installed so as to prevent the elastic unit  240  from coming off. 
     The operation of a fusing device having the above-described structure in an electrophotographic image forming apparatus will be described in detail with reference to the drawings. 
     Once electric power is supplied to the lead  116  of the heater  113  through the external lead  254 , the brush  220  and the electrode  210 , the electric power provokes heat radiation from the resistive coil  113   b . Some of the heat is transmitted to the fusing roller  112  through the first mica layer  113   a , and the rest is transmitted to the heat pipe  114  through the second mica layer  113   c . The working fluid  115  contained in the heat pipe  114  evaporates due to the transmitted heat. The heat of the vaporized working fluid  115  is transmitted to the fusing roller  112  through the first and second mica layers  113   a  and  113   c , respectively, formed on the surface of the heat pipe  114 . The fusing roller  112  receives the heat generated by the heater  113  and the heat of the working fluid  115  transmitted through the first and second mica layers  113   a  and  113   c  so that the surface temperature of the fusing roller  112  uniformly increases throughout the fusing roller  112  to a target temperature at which the toner  151  can be fused and fixed to the sheet  150 . 
     Thereafter, in a printing mode, the powder-state toner  151  is transferred to the sheet  150 , and is fused and fixed to the sheet  150  by the fusing roller  112  having a predetermined temperature while the sheet  150  passes between the fusing roller  112  and the pressing roller  190 . Then, the heat of the fusing roller  112 , which has fused and fixed the toner  151  to the sheet  150 , is taken away by the sheet  150 , so the working fluid  115  contained in the heat pipe  114  is liquefied. Thereafter, when heat is transmitted by the heater  113 , the working fluid  115  evaporates again. Consequently, the surface temperature of the fusing roller  112  is maintained at a target temperature appropriate for fusing and fixing the toner  151  so that the printing operation can be continued. 
     The target temperature for normal fusing and fixing of a toner image is 160-190EC. The fusing device  100  according to the present invention reaches the target temperature within about 10 seconds. After reaching the target temperature, the thermistor  118  measures the surface temperature of the fusing roller  112  so as to maintain the surface temperature of the fusing roller  112  within a predetermined range for normal fusing and fixing of the toner  151 . When the thermistor  118  fails to control the surface temperature and the surface temperature of the fusing roller  112  rapidly increases, the thermostat  119  mechanically cuts off the power of the power connecting unit  200  connected thereto, thereby preventing the surface temperature of the fusing roller  112  from rapidly increasing. Such a power supply operation can be changed depending on a setpoint of temperature. In addition, power supply can be controlled by ON/OFF control, a pulse width modulation method, or a proportional and integral (PI) method. 
     FIG. 10 is a perspective view of an example of the lead shown in FIG.  4 . Lead  170  is composed of a ring  172 , which surrounds the heat pipe  114 , and one side of which is electrically connected to an end of the resistive coil  113   b , and a string  174  extending from the ring  172 . The string  174  passes through the lead hole  122  of FIG. 7, and is connected to the electrode  210  of FIG.  9 . By using the lead  170 , breaking of the lead due to heat radiation from the lead in the air can be prevented, thereby more reliably connecting the electrode  210  to the heater  113 . 
     As described above, a fusing roller for an electrophotographic image forming apparatus according to the present invention uses a heat pipe, thereby reducing warming-up time for initial operation. Since a resistive coil is covered with an insulation layer, a heater can be easily manufactured. In addition, use of a heat sink and an insulation layer secures the reliability of the leads in the heater and end caps. 
     Although the preferred embodiments of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiment. Rather, various changes and modifications can be made within the spirit and scope of the present invention, as defined by the following claims.