Abstract:
A fixing section for use in an image forming apparatus, is provided with a heating member, a pressing member in which the heating member and the pressing member are arranged to form a nipping section therebetween and a belt member inserted through the nipping section and for fixing the toner image on the recording sheet by conveying the recording sheet through the nipping section; and a control section for obtaining recording sheet information regarding at least characteristics and a size and for controlling the fixing section. The pressing member comprises plural pressing heads differing in at least one of a physical characteristics and a shape from each others and the control section select one head of the plural pressing heads in accordance with the recording sheet information and control the fixing section to press the recording sheet through the belt member with the selected one head.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to an electrophotographic image forming apparatus equipped with a fixing device such as a copying machine, printer, facsimile, and so on. Specifically, this invention relates to an electrophotographic image forming apparatus equipped with a fixing device that fixes by clamping, pressing, and heating a paper sheet by two bodies of rotation.  
         [0002]     In general, a conventional fixing device fixes a toner image to a transfer sheet by letting a transfer sheet with a toner image pass through a nip area which is formed by two bodies of revolution (rollers) in pressure contact, heating, and pressing the toner image against the transfer sheet.  
         [0003]     When the image forming apparatus is of the color type, the transfer sheet is apt to have more toner and consequently it is apt to twine itself around the body of rotation when a toner image is heated and pressed. To prevent this, the nip area is dented in the side of the unfixed toner image. In this case, the roller which is in contact with the backside of the transfer sheet must be harder than the roller which is contact with the toner image side of the transfer sheet. In this case, the nip area is not wide enough for a poor-fixing transfer sheet such as a cardboard. Therefore, to fix such a poor-fixing transfer sheet, we must increase the fixing temperature or reduce the processing speed. As the result, the warm-up time becomes longer and the print productivity becomes lower. Additionally, after passing through the fixing device, such a hard transfer sheet may be curled to the nip shape. As the cardboard is hard and the nip area need not be convex in the side of the roller facing to the backside of the paper, it is possible to decrease the hardness of the roller in the backside of the transfer sheet and to make the nip area flat. However, in this status, a thin paper sheet may twine itself around the roller in the side of the unfixed toner.  
         [0004]     To solve the above problems, there have been disclosed various technologies such as a technology (e.g. Patent Document 1) that uses a plurality of rollers to select optimum conditions such as roller temperatures, diameters, circumferential speeds, and surface hardness according to water content and thickness of the transfer sheets and a technology (e.g. Patent Document 2) that select rollers according to the kinds of transfer sheets to suppress wrinkles of an envelope that holds a toner image and to assure the transparency of a color toner image on an OHT sheet (transparent sheet).  
         [0005]     Further, another technology (e.g. Patent Document 3) discloses a method of providing a roller to the unfixed toner image side of a transfer sheet, a belt to the opposite side of the transfer sheet, and a plurality of pressing members that press the belt against the roller, selecting one of the pressing members which have different lengths (widths) perpendicular to the movement of the transfer sheet, and causing the selected pressing member to press the belt against the roller with the pressing force changed. 
        Patent Document 1: Japanese Non-examined Patent Publication S54-95246     Patent Document 2: Japanese Non-examined Patent Publication H04-166878     Patent Document 3: Japanese Non-examined Patent Publication 2001-5312        
 
         [0009]     However, when some rollers are selected, their temperatures must be controlled simultaneously and the power consumption is required too much in the standby status. If this temperature control is omitted to suppress the standby power consumption, it takes much time before the selected rollers reach the preset control temperatures. In other words, it takes a lot of time for the first printout and the fixing may be insufficient. Further, if controlling is made to reduce the circumferential speeds of rollers, the print productivity becomes lower. Therefore, it is not enough to simply provide rollers that are different in temperature, diameter, circumferential speed, and surface hardness and to select them according to the operating conditions because of the long warm-up time after roller selection and the low print productivity. Furthermore, the technology disclosed by Patent Document 3 cannot assure the fixing and paper passing abilities of various kinds of transfer sheets under a changing print environment singly by changing the length (or width) of the pressing member perpendicular to the movement of the transfer sheet.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of this invention is to provide an image forming apparatus having a fixing device that can assure an overall fixing performance such as fixing ability, peeling ability, wrinkle-free properties, and optimization of temperature distribution.  
         [0011]     This purpose can be attained by the means below.  
         [0012]     An image forming apparatus having a fixing device for fixing a toner image onto a transfer sheet, comprising two bodies of rotation at least one of which is belt-shaped and pressed together to form a nip section, a heat source for heating at least one of the bodies of rotation, and a driving source for rotating at least one of two bodied of rotation to let a transfer sheet pass through the nip section and fix a toner image onto the transfer sheet, wherein the image forming apparatus further comprises 
        a plurality of pressing members for pressing the belt-shaped body of rotation against the other body of rotation,     a moving means for moving one of the pressing members towards the nip section and stopping there,     a control means for controlling movement and stopping of the pressing member, and     at least one of means for setting any of the size, type, brand, thickness, basis weight, smoothness, glossiness, and stiffness of the transfer sheet to be printed on an operation section, means for detecting any of the size, thickness, basis weight, smoothness, glossiness, and stiffness of the transfer sheet before fixing, and means for detecting the environmental temperature or humidity around the image forming apparatus and the temperature or water content of the transfer sheet and     one of the pressing members is moved to the nip section before the transfer sheet reaches the nip section.        
 
         [0018]     This invention can provide a fixing device that can assure the overall fixing performance by securing a pressing member that presses a transfer sheet against the roller via the belt and selecting a pressing member according to the condition of the transfer sheet. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic vertical sectional view of the whole image forming apparatus.  
         [0020]      FIG. 2  is an explanatory sectional view of the pressing member moving means.  
         [0021]     FIGS.  3 ( a ) to  3 ( d ) each shows details of the pressing pad.  
         [0022]     FIGS.  4 ( a ) to  4 ( d ) each shows an example of a detecting means that detects a condition related to the transfer sheet before transferring.  
         [0023]     FIGS.  5 ( a ) and  5 ( b ) each shows part of an operation panel provided on the top of the image forming apparatus.  
         [0024]      FIG. 6  shows a mechanism that places the pressing means inside the heating belt and pressing the belt against the pressing roller. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     First we explain a fixing device in accordance with this invention and an image forming apparatus which is equipped therewith.  
         [0026]     It is to be understood that the description of embodiments below is not intended to limit the technical range of this invention by terms in the description.  
         [0027]      FIG. 1  is a schematic vertical sectional view of the whole image forming apparatus.  
         [0028]     In  FIG. 1 , the major components are a photosensitive member  10 , a Scorotron charger  11  as a charging means, an image writer  12  as an image writing means, a developer  13  as a developing means, a cleaning device  14  for cleaning the surface of the photosensitive member  10 , a cleaning blade  15 , a developing sleeve  16  and an intermediate transfer belt  20 . The image forming apparatus  1  consists of the photosensitive member  10 , the Scorotron charger  11 , the developer  13 , the cleaning device  14 , and so on. The image forming means  1  of four colors (yellow Y, magenta M, cyan C, and black K) are the same in mechanical configuration. So, in  FIG. 1 , the reference characters are assigned only for the configuration of the yellow image forming means as the representative.  
         [0029]     The image forming means  1  of four colors (yellow Y, magenta M, cyan C, and black K) are provided in that order of Y, M, C, and K along the movement of the intermediate transfer belt  20 . The photosensitive members  10  are respectively in contact with the tensioned surface of the intermediate transfer belt  20  and rotate there in the same direction as the movement of the intermediate transfer belt  20  at the same line speed.  
         [0030]     The intermediate transfer belt  20  are supported and tensioned by a driving roller  21 , a grounding roller  22 , a tension roller  23 , a neutralization roller  27 , and a driven roller  24 . A belt unit  3  consists of these rollers, the intermediate transfer belt  20 , a transfer device  25 , and a cleaning device  28 .  
         [0031]     The intermediate transfer belt  20  is driven by the rotation of a driving roller  21  by a driving motor (which is not shown in the figure).  
         [0032]     The photosensitive member  10  is made of a cylindrical metallic base such as an aluminum cylinder which has a photoconductive layer such as an electroconductive layer, a-Si layer or an organic photosensitive layer (OPC) on its circumferential surface and rotates counterclockwise (in the arrow direction of  FIG. 1 ) with the conductive layer. grounded.  
         [0033]     An electric signal corresponding to the image data sent from an image reader  80  is converted into an optical signal by an image formation laser and the optical signal is projected to the photosensitive member  10  by the image writer  12 .  
         [0034]     The developer  13  is equipped with a cylindrical non-magnetic stainless-steel or aluminum developing sleeve  16  which is at a preset space away from the circumference of the photosensitive member  10  and rotates in the same direction as the photosensitive member  10  at the most closest position.  
         [0035]     The intermediate transfer belt  20  is an endless belt of a volume resistivity of 10 6  to 10 12  Ω·cm. It is a semi-conductive seamless belt of 0.015 to 0.05 mm thick prepared by dispersing a conductive material in engineering plastic such as modified polyimide, thermosetting polyimide, ethylene-tetrafluoro-ethylene copolymer, vinylidene polyfluoride, and nylon alloy.  
         [0036]     The transfer device  25  has a function of transferring a toner image from the photosensitive member  10  onto the intermediate transfer belt  20  when receiving a d.c. current of a polarity opposite that of the toner. The transfer device  25  can be a corona discharger or a transfer roller.  
         [0037]     The transfer roller  26  can move to touch or detach from the grounding roller  22  and transfer the toner image from the intermediate transfer belt  20  to a transfer sheet P.  
         [0038]     The cleaning device  28  is provided opposite the driven roller  24  with the intermediate transfer belt  20  therebetween. After the intermediate transfer belt  20  transfers the toner image onto the transfer sheet P, the charge of toner left on the transfer belt  20  is weakened by the neutralization roller  27  which has an a.c. voltage superimposed with a d.c. voltage whose polarity is opposite the polarity of the toner. Then the toner on the surface of the transfer belt  20  is scraped away by the cleaning blade  29 . The fixing device  4  in accordance with this invention will be explained in detail below.  
         [0039]     The other components are paper pickup rollers  70 , timing rollers  71 , paper cassettes  72 , paper feed rollers  73 , an operation panel  85 , and a controller B 1  as a control means.  
         [0040]     Below will be explained the fixing device  4  in accordance with this invention.  
         [0041]      FIG. 2  is an explanatory sectional view of the pressing member moving means.  
         [0042]     In  FIG. 2 , the heating roller  41  is a cylindrical aluminum mandrel  413  coated with an elastic heat resisting layer  412  and an outer separation layer  411 . The heating roller  41  is heated to a preset temperature by a halogen heater  46  as a heating source in the hollow part of the heating roller  41 . The temperature is detected by a non-contact temperature sensor  414  provided near the surface of the heating roller  41  and sent to the controller B 1 . The controller B 1  controls the surface temperature of the heating roller  41  to a preset temperature by turning on and off the halogen heater  46 .  
         [0043]     The pressing belt  47  is a polyimide belt coated with a silicone rubber layer and a thin PFA resin layer. When a transfer sheet P having a toner image comes into the nip section which is a fixing area by means of the paper guides and the like, the pressing belt  47  and the pressing pad (pressing head) A 1  catch and press the transfer sheet P against the heating roller  41  to fix the toner image onto the transfer sheet P.  
         [0044]     The pad moving mechanism  42  consists of a cylindrical pad supporting roller  420 , and pressing pads (A 1 , A 2 , A 3 , and A 4 ). The rigid pad supporting roller  420  made of a rigid material has a plurality of longitudinal grooves M to hold the pressing pads (pressing heads) (A 1 , A 2 , A 3 , and A 4 ).  
         [0045]     At least one of the pressing pads (A 1 , A 2 , A 3 , and A 4 ) is different from the other pressing pads in hardness, heat capacitance, thickness or heat capacitance distribution along and perpendicular to the movement of the pressing belt.  
         [0046]     The heat conductivity of the pad supporting roller  420  is preferably low. The pad supporting roller  420  is mounted on a rotary shaft  425  which is driven by a driving section (which is not shown in the figure).  
         [0047]     By an instruction of the controller B 1 , the rotary shaft  425  is rotated a preset angle to move any of the pressing pads (A 1  to A 4 ) to the heating roller  41 , stopped and held at a preset position to press the heating roller  41  and form a nip section T.  
         [0048]     Therefore, the nip sections T formed by respective pressing pads (A 1  to A 4 ) are different in pressure, nip length, and fixing condition.  
         [0049]     Although this embodiment uses four pressing pads (A 1  to A 4 ), four or more pressing pads can be used.  
         [0050]     Referring to  FIG. 1 , still other components are guide plates G, a belt driving roller  43 , a tension roller  44 , a driven roller  45 , a halogen heater  46 , and ejection rollers  48 .  
         [0051]      FIG. 3  shows details of the pressing pad.  
         [0052]     In  FIG. 3 ( a ), the pressing pad (A 1  to A 4 ) is an elastic silicone rubber member  422  coated with Teflon®-related sliding sheet  423 .  
         [0053]     At least one of the pressing pads (A 1  to A 4 ) is different from the other pressing pads in hardness, heat capacitance, thickness “t” along the movement of the transfer sheet, thickness “h” perpendicular to the movement of the transfer sheet, distribution of thickness “h” perpendicular to the movement of the transfer sheet, distribution of heat capacitance perpendicular to the movement of the transfer sheet, and distribution of hardness perpendicular to the movement of the transfer sheet.  
         [0054]     The base of the elastic member  422  is low heat conduction silicone rubber of a heat conductivity of 0.05 to 0.25 W/m·k and coated with a sliding sheet  423  made from Teflon®-related plastic resin (PTFE, etc.) to reduce the friction between the pressing belt  47  and the elastic member.  
         [0055]     As shown in  FIG. 3 ( b ), it is possible to make the pressing pads (A 1  to A 4 ) thicker in the center “h” (than the ends). Further as shown in  FIG. 3 ( c ), it is possible to divide the pressing pad in one groove into a plurality of pieces (1 to n), make the pieces  422  higher in hardness towards the center of the groove (or lower towards the outer ends of the pad). Furthermore as shown in  FIG. 3 ( c ), it is possible to divide the pressing pad in one groove into a plurality of pieces (1 to n), make the pieces  422  lower in heat conductivity towards the center of the groove (or higher towards the outer ends of the pad). Still further, as shown in  FIG. 3 ( d ), it is possible to curve the groove M and make the pressing pad thicker in the center “h” (or lower in the ends).  
         [0056]     The pressing pads (A 1  to A 4 ) of these different configurations are respectively bonded to the grooves, selected and moved under a selected condition (size, type, brand, thickness, basis weight, smoothness, glossiness, and stiffness of the transfer sheet to be printed) when the condition is preset on the operation section.  
         [0057]     When a condition (size, thickness, basis weight, smoothness, and glossiness of the transfer sheet) is detected before image transferring and the result of detection is sent to the control section B 1  in advance, a pressing pad satisfying the condition is selected.  
         [0058]     Further, when an environmental temperature or humidity of the image forming apparatus and the temperature or water content of the transfer sheet is detected and the result of detection is sent to the control section B 1  in advance, a pressing pad satisfying the condition is selected.  
         [0059]     The optimum fixing is enabled by the nip section T formed by the selected pressing pad (A 1  to A 4 ) and the heating roller  41 .  
         [0060]     A program created by experimental data is used to select a pressing pad that satisfies a condition (size, quality, brand, thickness, basis weight, smoothness, glossiness, stiffness, environmental temperature or humidity, and temperature or water content of the transfer sheet). The program is stored in the control section B 1 .  
         [0061]      FIG. 4  shows an example of a detecting means that detects a condition related to the transfer sheet before transferring.  
         [0062]      FIG. 4 ( a ) is a detecting means that measures the thickness of the transfer sheet P. In  FIG. 4 ( a ), the ends of the roller  74   a  are supported by bearings provided on the frame which is not shown in the figure. The roller  74   b  is a displacement roller which is supported by bearings to move perpendicularly to the movement of the transfer sheet. The sensor S 1  is a displacement detection sensor such as an ultrasonic sensor S 1 .  
         [0063]     When the transfer sheet P is clamped and carried by the rollers ( 74   a  and  74   b ), the roller  74   b  moves from the dotted-line position to the solid-line position by the thickness “e” of the transfer sheet P. The displacement sensor S 1  detects this displacement and sends the displacement information to the control section B 1 . The control section B 1  selects a pressing pad fit for the thickness.  
         [0064]      FIG. 4 ( b ) shows a detector that measures the smoothness and the glossiness of the transfer sheet P. In  FIG. 4 ( b ), the sensor S 2  detects the quantity of light reflected on the transfer sheet P to measure the roughness and glossiness of the surface of the transfer sheet, and sends its information to the control section B 1 . The control section B 1  selects a pressing pad fit for the roughness and glossiness.  
         [0065]      FIG. 4 ( c ) shows a detector that measures the stiffness of the transfer sheet P. In  FIG. 4 ( c ), a pair of rollers  75  in the delivery path transfer the transfer sheet P. The sensor S 3  for detecting the quantity of light reflected on the transfer sheet is provided a preset distance “f” from the delivery roller pair  75 .  
         [0066]     The transfer sheet P is clamped and carried by the delivery roller pair  75  and its leading edge is detected.  
         [0067]     The sheet P warps much if the stiffness of the transfer sheet P is low or small if the stiffness of the transfer sheet P is high. The quantity of light that the sensor S 3  receives is dependent upon the magnitude of this warp. The sensor S 3  detects the stiffness of the transfer sheet from the relationship between the light quantity and the warp magnitude and sends the result of detection to the control section B 1 . The control section B 1  selects a pressing pad fit for the stiffness.  
         [0068]      FIG. 4 ( d ) shows a detector for measuring the water content of the transfer sheet. In  FIG. 4 ( d ), the roller pair  77  is a pair of conductive delivery rollers to clamp and carry the transfer sheet. As a voltage E is applied to this roller pair  77 , the resistance between the rollers (equivalent to the paper resistance) becomes low and the current A becomes greater when the water content of the transfer sheet is high. Contrarily, when the water content of the transfer sheet is low, the resistance becomes high and the current A becomes lower. The information of this current A is sent to the control section B 1 , and the control section B 1  selects a pressing pad fit for the water content.  
         [0069]     Additionally, a sensor for detecting the environmental temperature or humidity of the image forming apparatus is provided inside near the casing of the image forming apparatus and a sensor for detecting the temperature of the transfer sheet is provided in the paper feed section. Their information is sent to the control section B 1  and used to select an optimum pressing pad. The casing of the image forming apparatus has apertures (narrow enough to prevent invasion of fingers) near the sensor for detecting the environmental temperature or humidity of the image forming apparatus.  
         [0070]     The transfer sheet sizes can be automatically detected by a well-known means in a paper cassette  72 .  
         [0071]      FIG. 5  shows part of an operation panel provided on the top of the image forming apparatus.  
         [0072]     The operation panel has a paper property selection field.  
         [0073]      FIG. 5 ( a ) shows a list of paper property items to be selected.  
         [0074]      FIG. 5 ( b ) shows an example of paper property items.  
         [0075]     As already explained, paper properties can be detected and selected by sensors provided in the paper feed and delivery paths. Further there has been a method of enabling the operator to enter paper properties and controlling selection of an optimum pressing pad. This method will be explained in detail below.  
         [0076]     In  FIG. 5 , the operation panel  85  has a paper property selection field  851  which is a means to set paper properties.  
         [0077]     When the operator sets any paper properties (size, type, brand, thickness, basis weight, smoothness, and glossiness of the transfer sheet) on the paper property selection field  851 , the control section B 1  selects a pressing pad that satisfies the preset condition.  
         [0078]     Although the above embodiment uses a roller as a body of rotation that is in contact with unfixed toner and a belt as another body of rotation that is in contact with the backside of the transfer sheet, the configuration of  FIG. 6  can attain the effect of this invention.  
         [0079]      FIG. 6  shows a mechanism that places the pressing means inside the heating belt and pressing the belt against the pressing roller.  
         [0080]     The fixing method of  FIG. 6  uses a heating roller  41 A, a belt driving roller  43 , a tension roller  45 , heats the heating belt  47 A, presses the transfer sheet P with a toner image against the heating roller by the pressing means  43 A and heats the transfer sheet. Two halogen heaters ( 46 A and  46 B) are controlled individually to turn on and off by the control section B 1  according to the outputs of the temperature sensors ( 415  and  414 ). The belt heating roller  41 A is made of a cylindrical aluminum mandrel coated with fluorine resin or the like. Its configuration is basically the same as that of  FIG. 2  and its explanation is omitted.  
         [0081]     A program that enables the operator to enter property values (thickness, basis weight, smoothness, glossiness, and so on) is stored in the control section B 1 . By entering values using the ten-key pad  852  (see  FIG. 5 ), the operator can select a pressing pad fit for the preset condition.