Abstract:
A fixing device, including a fixing rotary body including a first heat source, a pressing rotary body including a second heat source, and a control device configured to cause, at a time of warm-up of the fixing device, the fixing rotary body to rotate while causing at least one of the first heat source and the second heat source to generate heat, and to allow, after a surface temperature of the fixing rotary body has risen to a preselected warm-up temperature, a sheet pass or a printing to be executed and cause, if a sheet pass is not executed, the fixing rotary body to rotate for a preselected period of time.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a fixing device for use in an image forming apparatus and including a fixing roller, a press roller, a heat roller having a heat source thereinside and a fixing belt passed over the fixing roller and press roller. 
   2. Description of the Background Art 
   A fixing device applicable to an image forming apparatus is disclosed in, e.g., Japanese Patent Application No. 2003-307963. The fixing device taught in this document is configured to achieve various purposes including reducing a warm-up time to a stand-by state and a buildup time from the stand-by state, speeding up fixation, stabilizing the output of a power supply, and reducing power consumption. The document proposes an image forming apparatus using such a fixing device also. A controller included in the fixing device controls the turn-on and turn-off of the individual heater connected to a power supply. More specifically, at the time of warm-up, buildup and printing, the controller turns on only the heater of the heat roller to thereby apply a sufficient amount of heat to the fixing belt, which is turning, via said heat roller. Further, in a stand-by state, the controller turns on the heaters of the heat roller and press roller in order to maintain the temperature of the heat roller and press roller. In this manner, the controller of the above fixing device turns on or turns off each heater at a particular timing. 
   Generally, in a conventional fixing device, a fixing roller and a press roller rotatable in pressing contact therewith each are provided with a surface layer formed of rubber, but a heat roller, accommodating a heater therein, is not provided with such a surface layer in order to have a small thermal capacity. While sharp thermal response is achievable if the heat roller is provided with a small thermal capacity small and if the heater of the heat roller is caused to generate a great amount of heat in a continuous sheet-pass mode, this scheme is unable to reduce power consumption. To reduce the power consumption of the fixing device and therefore the total power consumption of an entire image forming apparatus, the amount of heat to be generated by each of the heaters of the fixing roller and press roller is made smaller than the amount of heat to be generated by the heater of the heat roller. 
   However, the problem with the conventional fixing device stated above is that the heat roller, having a smaller thermal capacity than the fixing roller and press roller, is greatly effected by the temperatures of the fixing roller and press roller via the fixing belt at the beginning of a sheet-pass after the stand-by mode. More specifically, if the temperature of the fixing roller or that of the press roller is lowered in, e.g., the stand-by mode in which the fixing belt remains in a halt, the temperature of the fixing belt is low at the beginning of a sheet pass and therefore degrades fixation. 
   In light of the above, it is necessary to maintain the temperatures of the fixing roller and press roller above a preselected temperature in the stand-by mode. However, although the fixing roller and press roller both receive heat from the fixing belt via their surface layers at the time of start-up of the fixing device because the belt is turned, the temperatures of the fixing roller and press roller are lowered after the stop of movement of the belt partly because the amount of heat output from the heater is small and partly because the fixing roller and press roller each have a great thermal capacity. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a fixing device capable of reducing a warm-up time and preventing the temperature of a fixing belt from being lowered at the beginning of a sheet path just after warm-up, and an image forming apparatus using the same. 
   A fixing device of the present invention includes a fixing rotary body including a first heat source, a pressing rotary body including a second heat source, and a controller. The controller causes, at the time of warm-up of the fixing device, the fixing rotary body to rotate while causing at least one of the first and second heat sources to generate heat and allows, after the surface temperature of the fixing rotary body has risen to a preselected warm-up temperature, a paper pass or a printing to be executed or causes, if a sheet pass is not executed, the above one heat source to stop generating heat and then causes the fixing rotary body to rotate for a preselected period of time. 
   Further, a fixing device of the present invention includes a fixing roller, a press roller, a heat roller including a heat source, a fixing belt passed over the fixing roller and heat roller, and a controller. At least one of the fixing roller and press roller includes a heat source. The controller causes, at the time of warm-up of the fixing device, the fixing belt to turn while causing at least one of heat sources included in rollers to generate heat and allows, after the surface temperature of the fixing belt has risen to a preselected warm-up temperature, a paper pass or a printing to be executed or causes, if a sheet pass is not executed, the above one heat source to stop generating heat and then causes the fixing belt to turn for a preselected period of time. 
   An image forming apparatus, including either one of the fixing devices stated above, is also disclosed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a view showing an image forming apparatus embodying the present invention; 
       FIG. 2  is a fragmentary view showing a fixing device included in the illustrative embodiment; 
       FIG. 3  is an enlarged section showing essential part of the fixing device; 
       FIGS. 4 through 7  show curves representative of a relation between the temperature of a fixing belt included in the fixing device of the illustrative embodiment and time; 
       FIGS. 8A and 8B  are flowcharts demonstrating a specific operation of the illustrative embodiment; 
       FIG. 9  shows an alternative embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  of the drawings, an image forming apparatus embodying the present invention is shown and may be implemented as, e.g., an electrophotographic copier or a printer by way of example. As shown, the image forming apparatus is generally made up of an document scanning unit  11  for reading documents, a printer or image forming section  12  for forming images, an ADF (Automatic Document Feeder)  13 , a document stack tray for stacking documents sequentially driven out of the ADF  13 , a sheet feeding section  19  including sheet cassettes  15  through  18 , and a sheet stack tray  20  for stacking paper sheets or similar recording medium driven out of the printer  12 . 
   In operation, the operator of the image forming apparatus stacks desired documents D on a tray  21  included in the ADF  13  and then presses a print start key or otherwise manipulates an operating section not shown. In response, the top document D is paid out by a pickup roller  22  in a direction indicated by an arrow B 3  in  FIG. 1  and then conveyed by a belt  23  in rotation to a glass platen  24 , which is fixed on the top of the image scanner  11 . When the document D is brought to a stop on the glass platen  24 , an image scanner  25 , positioned between the printer  12  and the glass platen  24  reads the image of the document D. More specifically, the image scanner  25  includes a light source  26  for illuminating the document D positioned on the glass platen  24 , and optics  27  for focusing a document image on a CCD (Charge Coupled Device) image sensor or similar image sensor, or photoelectric converter, 28. 
   After the document D has been fully scanned by the image scanner  25 , it is again conveyed by the belt  23  to the document stack tray  14  in a direction indicated by an arrow B 2  in  FIG. 1 . In this manner, the documents D are sequentially fed to the glass platen  24  one by one and read by the document scanning unit  11  in which the document scanner  25  is arranged. 
   The printer  12  includes a photoconductive drum  30 , which is a specific form of an image carrier. The photoconductive drum (simply drum hereinafter)  30  is rotated clockwise, as viewed in  FIG. 1 , while having its surface uniformly charged to a preselected potential by a charger  31 . An optical writing unit  32 , positioned in the upper portion of the printer  12 , emits a laser beam L modified in accordance with image data read by the image scanning unit  11 . The laser beam L scans the charged surface of the drum  30  imagewise to thereby form a latent image on the drum  30 . A developing unit  33  develops the latent image with toner when the latent image is conveyed thereby by the drum  20  in rotation, thereby forming a corresponding toner image. An image transferring unit  34 , facing the drum  30 , transfers the toner image thus formed on the drum  20  to a paper sheet or similar recording medium P fed from the sheet feeding section  19  in the direction B 3 . A drum cleaner  35  removes toner left on the drum  30  after the image transfer. 
   More specifically, the sheet feeding section  19 , positioned in the lower portion of the printer  12 , includes a plurality of (four in the illustrative embodiment) sheet cassettes  15 ,  16 ,  17  and  18  each being loaded with a stack of paper sheets P. A sheet P stacked on the top of any one of the sheet cassettes  15  through  18  is paid out in the direction B 3 , so that the toner image is transferred from the drum  30  to the paper sheet P. The paper sheet P, carrying the toner image thereon, is conveyed to a fixing device  100 , which is also disposed in the printer  12 , in a direction indicated by an arrow B 4  in  FIG. 1 . The fixing device  100  fixes the toner image on the paper sheet P with heat and pressure, as will be described specifically hereinafter. Subsequently, the paper sheet P is driven out of the printer  12  to the sheet stack tray  20  via an outlet roller pair  37 . 
   Reference will be made to  FIG. 2  for describing the configuration of the fixing device  100  unique to the illustrative embodiment. As shown, the fixing device  100  includes a fixing roller or fixing member  50 , a heat roller  60 , a press roller  70  and a fixing belt  40  passed over the fixing roller  50  and heat roller  60 . 
   The fixing roller  50  is caused to rotate by a motor  90  to, in turn, drive the heat roller  60  and press roller  70  via the fixing belt  40 . Temperature sensors  53 ,  63  and  73  adjoin the fixing roller  50 , heat roller  60  and press roller  70 , respectively, each for sensing the surface temperature of the associated roller. Further, heat sources  55 ,  65  and  66  and  75  are disposed in the fixing roller  50 , heat roller  60  and press roller  70 , respectively. A controller or control means  80  controls the motor  90  and heat sources  55 ,  65  and  66  and  75  in accordance with the outputs of the temperature sensors  53 ,  63  and  73 . 
   The fixing roller  50  is made up of a metallic core  51  and a cover layer  52  covering the core  51 . In the illustrative embodiment, the heat source or heater  55  is disposed in the core  51 . The cover layer  52  is formed of rubber or similar elastic material on which a fluorocarbon resin layer is formed. Likewise, the heat roller  60  includes a metallic core  61  in which the heat sources or heaters  65  and  66  are disposed. The fixing belt  40  passed over the fixing roller  50  and heat roller  60  is implemented by an Ni, SUS steel or similar metallic film or a Pl, PAl or similar resin layer and a fluorocarbon resin layer formed on the surface thereof. 
   The press roller  70  is rotatable while being pressed against the fixing roller  50  via the fixing belt  40 . The press roller  70 , like the fixing roller  50 , is made up of a metallic core  71  and a cover layer  72  covering the core  71 . In the illustrative embodiment, the heat source or heater  75  is disposed in the metallic core  71 . The cover layer  72  is formed of rubber or similar elastic material on which a fluorocarbon resin layer is formed. 
   In the illustrative embodiment, the controller  80  selectively turns on or turns off the heaters  55 ,  65  and  66  and  75  of the fixing roller  50 , heat roller  60  and press roller  70 , respectively, in accordance with temperatures sensed by the temperature sensors  53 ,  63  and  73  and the drive timing of the image forming apparatus. The paper sheet P, not shown, is conveyed via a nip N between the cover layers  52  and  72  of the fixing roller  50  and press roller  70 , respectively, which are pressed against each other. As a result, the toner image carried on the paper sheet P is fixed by the heat and pressure of the fixing roller  50  and fixing belt  40 . While the fixing roller  50  and press roller  70  both are provided with a heater in the illustrative embodiment, only one of them may be provided with a heater, if desired. 
   In the illustrative embodiment, the total amount of heat generated by the heaters  65  and  66  of the heat roller  60  is selected to be greater than the amount of heat generated by the heater  55  of the fixing roller  50  or the heater  75  of the press roller  70 . This is because the cover layers  52  and  72 , respectively included in the fixing roller  50  and press roller  70 , are low in thermal conductivity and therefore obstruct the rapid warm-up of the rollers  50  and  70  after the start-up of the image forming apparatus. To rapidly warm-up the fixing belt  40  at the time of start-up in such conditions, it is desirable to rotate the heat roller  60  not coated with silicone rubber. 
   If the fixing belt  40 , heated to a preselected temperature after the start-up of the apparatus, is caused to stop turning, then the temperature of the fixing roller  50  and that of the press roller  70  drop because the rollers  50  and  70  are not fully warmed up to the inside then. Therefore, if the fixing belt  40  is again driven to convey a paper sheet in the above condition, then the heat of the belt  40  is rapidly absorbed by the fixing roller  50  and press roller  70 . It is therefore likely that heat output from the heat roller  60  is too short to maintain a fixable temperature. As a result, the heaters  55  and  75  of the fixing roller  50  and press roller  70 , respectively, each are required to generate a greater amount of heat at the time of start-up of the apparatus. 
   More specifically, as shown in  FIG. 3 , if the fixing belt  40 , warmed up to the preselected temperature at the time of start-up of the apparatus, is caused to stop turning, the fixing roller  50  and press roller  70  are respectively heated by the heaters  55  and  75  in zones  52   a  and  72   a  thereof, but not heated in zones  52   b  and  72   b  between the surfaces of the rollers  50  and  70  heated by the fixing belt  40  and the zones  52   a  and  72   a , respectively. Consequently, if amounts of heat for heating the above zones  52   b  and  72   b  are not fed from the heaters  55  and  75 , respectively, then the temperature of the entire fixing roller  50  or that of the entire press roller  70  drops, lowering the temperature of the nip N below the fixable temperature when a paper sheet is passed. 
   By contrast, if the fixing belt  40 , warmed up to the fixable level or reached a ready state, is caused to continuously turn, then it maintains the surfaces of the fixing roller  50  and press roller  70  at a preselected temperature. Even when the paper sheet P is passed through the nip N between the fixing roller  50  and the press roller  70  in the above condition, the temperature at the nip N can be maintained by the heat of the heaters  65  and  66  because the thermal capacity of a single paper sheet P and therefore the temperature drop of the nip N is negligible. 
   More specifically, as shown in  FIG. 4 , assume that the temperature of the fixing belt  40  is raised to a preselected warm-up or fixable temperature and then caused to stop rotating. Then, the temperatures of the fixing roller  50  and press roller  70  drop for a moment and again rise to the fixable temperature, as indicated by a dashed curve in  FIG. 4 . On the other hand, when heat is transferred from the heat roller  60  to the fixing belt  40  that is continuously turning, the temperature of the belt  40  is prevented from dropping, as indicated by a solid curve in  FIG. 4 . 
   If a printing cycle is not started just after the warm-up to the ready state, then the fixing device  100  simply idles at a controlled temperature higher than the preselected warm-up temperature or similar preselected sheet-pass temperature. Therefore, the zones  52   b  and  72   b  of the fixing roller  50  and press roller  70 , respectively, not fully heated by the fixing belt  40  are rapidly heated by the respective heaters  55  and  57  and fixing belt  40 . Consequently, even if the fixing belt  40  is caused to stop turning on the elapse of a preselected period of time, the temperature of the fixing roller  50  and press roller  70  can be raised to the temperature assigned to the ready or stand-by state without being lowered. 
   Further, in the illustrative embodiment, assume that a print command is input during idling performed in the ready state after the start-up of the apparatus. Then, if the temperatures of the fixing roller  50  and press roller  70  are sharply lowered in the event of transition to the stand-by temperature just after a sheet pass, then the above temperatures are apt to become lower than the fixable temperature at the beginning of feed of the next paper sheet. In light of this, in the illustrative embodiment, idling is continued even after a sheet pass for thereby preventing the temperatures of the fixing roller  50  and press roller  70  from being lowered. For this purpose, the controller  80  sets a total duration of idling beforehand and adds each sheet-pass time thereto. This is successful to reduce a period of time necessary for a warm-up and to prevent the temperature of the fixing belt  40  from dropping just after a start-up at the same time. 
   In the illustrative embodiment, the controller  80  does not maintain any one of the heaters turned on for a long period of time by controlling the temperature during idling above a warm-up control temperature or similar sheet-path temperature set beforehand inclusive. Therefore, as shown in  FIG. 5 , the controller  80  is capable of smoothly controlling the temperature just after the elevation to the warm-up temperature. Also, because the other units of the image forming apparatus are not operating, it is possible to turn on the heaters  55  and  75  of the fixing roller  50  and press roller  70 , respectively, with limited power and to stop the idling in a short period of time. 
   In the illustrative embodiment, because the duration of idling is extended, the heaters  55  and  75  of the fixing roller  50  and press roller  70 , respectively, each should only output an amount of heat sufficient to maintain the temperature in the stand-by or ready state. This allows power thus saved to be fed to the heaters  65  and  66  of the heat roller  60  for thereby rapidly heating the fixing belt  40 . More specifically, as shown in  FIG. 6 , for given conditions, the fixing belt  40  can be maintained at a higher temperature when preselected power is applied to the heaters  65  and  66  (solid curve) than when the same power is applied to the heaters  55  and  75  (dashed curve). 
   In the illustrative embodiment, the heat roller  60  is bare, i.e., not provided with a cover layer and can therefore efficiently heat the fixing roller  50  via the fixing belt  40 . Therefore, idling serves to maintain the temperature elevation characteristic of the fixing belt  40  from the time of the start-up of the apparatus to the beginning of a sheet pass desirable even if the amount of power consumption by the heaters  65  and  66  of the heat roller  60  is reduced. Consequently, for a given amount of power consumption, the fixing belt  40  can be efficiently heated even if greater power is applied to the heaters  65  and  66  than to the heaters  55  and  75 .  FIG. 7  compares, for a given total amount of power selected beforehand, a case wherein great power is applied to the heaters  65  and  66  of the heat roller  60  (solid line) and a case wherein greater power is applied to the heaters  55  and  75  of the fixing roller  50  and press roller  70 , respectively, (dashed line). As shown, the case indicated by the solid line allows the temperature of the fixing belt  40  to rise more sharply than the case indicated by the dashed line. 
   A specific operation of the controller  80  included in the illustrative embodiment will be described with reference to  FIGS. 8A and 8B . As shown in  FIG. 8A , after the start of warm-up of the fixing device  100 , the controller  80  determines whether or not the temperatures of the rollers  50 ,  60  and  70  all are lower than a set temperature A inclusive (step S 1 ). The set temperature A is used to determine whether or not the rollers are rotated at the time of warm-up. If the answer of the step S 1  is positive (Yes), meaning that the temperatures of the rollers  50 ,  60  and  70  are lower than the set temperature A inclusive, then the controller  80  turns on the heaters  55 ,  65  and  66  and  75  of the rollers  50 ,  60  and  70 , respectively, (step S 2 ) and starts turning the fixing belt  40  (step S 3 ). After the step S 3 , the controller  80  determines whether or not the rollers  50 ,  60  and  70  all are lower than a set temperature B representative of the end of warm-up of the rollers inclusive (step S 4 ). If the answer of the step S 4  is Yes, then the controller  80  repeats the steps S 1  through S 4  until the rollers  50 ,  60  and  70  reach the set temperature B. 
   On the other hand, if the temperatures of the rollers  50 ,  60  and  70  are higher than the set temperature A (No, step S 1 ) and lower than the set temperature B (Yes, step S 5 ), then the controller  80  turns on the heaters  55 ,  65  and  66  and  75  of the rollers  50 ,  60  and  70 , respectively, until the rollers  50 ,  60  and  70  all reach the set temperature B (step S 6 ). When the temperatures of the rollers  50 ,  60  and  70  all become higher than the set temperature B (No, step S 5 ), then the controller  80  brings the fixing device  100  into a ready or stand-by state (step S 7 ), causes the fixing belt  40  to stop rotating (step S 8 ), and controls the stand-by time of the heaters  55 ,  65  and  66  and  75  of the rollers  50 ,  60  and  70 , respectively, (step S 9 ). 
   If the temperatures of the rollers  50 ,  60  and  70  are higher than the set temperature B (No, step S 4  or No, step S 5 ), the controller  80  establishes the ready state.  FIG. 8B  shows a procedure to be executed in the ready state. As shown, while controlling the rollers  50 ,  60  and  70  to the set temperature B (step S 11 ), the controller  80  sums up the durations of rotation effected after establishing the ready state (step S 12 ) and then determines whether or not the total duration of rotation is shorter than a preselected period of time T inclusive (step S 13 ) If the answer of the step S 13  is No, meaning that the total duration of rotation is longer than the preselected period of time T, then the controller  80  determines whether or not a print start command is input (step S 14 ). In response to the print start command (Yes, step S 14 ), the controller  80  stops counting the total duration of rotation (step S 15 ), executes sheet-pass control with the rollers  50 ,  60  and  70  (step S 16 ), ends the sheet-pass control (step S 17 ), and then returns to the step S 11 . 
   By executing the control stated above with reference to  FIGS. 8A and 8B , the illustrative embodiment is capable of reducing the warm-up time of the fixing device  100  after the startup of the apparatus and preventing the temperature of the fixing belt  40  from dropping at the time of beginning of a sheet pass just after the warm-up at the same time. It should be noted that the belt  40  does not have to be idled in the event of a sheet pass effected after the stand-by state, the controller  80  determines whether or not the fixing belt  40  should be turned by using a preselected threshold temperature. 
     FIG. 9  shows an alternative embodiment of the fixing device in accordance with the present invention. As shown, the fixing device, generally designated by the reference numeral  200 , includes a fixing belt  240  provided with a heat generating layer, not shown, formed of metal. The fixing belt  240  is heated by an electromagnetic induction heating device  270  made up of a coil  271  and a core  272 . A high-frequency voltage is applied from a high-frequency current source, not shown, to the coil  271  so as to form a magnetic field, so that a current is induced in the heat generating layer of the fixing belt  240  for thereby heating the fixing belt  240 . The illustrative embodiment is capable of heating the fixing belt  240  more efficiently than the previous embodiment. 
   While the fixing belt  240  of the illustrative embodiment is directly heated, an arrangement may alternatively be made such that a drum  260 , formed of iron, nickel or similar metal, generates heat. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.