Abstract:
An image forming apparatus including: an image bearing body; an image forming section to form a toner image on the image bearing body; a transfer section to transfer the toner image on a recording material; and a fixing apparatus having a pressure member and a heating member, which fixes the toner image on the recording material transported their between, wherein the fixing apparatus comprises a cooling section to cool the pressure member, and a controlling section to control the cooling section according to an interval between recording materials.

Description:
[0001]     This application is based on Japanese Patent Application No. 2005-155125 filed with Japan Patent Office on May 27, 2005, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to image forming apparatuses such as copying apparatuses, printers, etc., that carry out image forming using the electrophotographic method, and in particular, to image forming apparatuses that carry out fixing of unfixed toner images using a fixing unit by applying heat and pressure.  
         [0004]     2. Description of Related Art  
         [0005]     In an image forming apparatus, the toner images formed on an image bearing body (photoreceptor) or on an intermediate transfer body are transferred by an image transfer section on to a recording material (also called a transfer material), and the recording material carrying the toner image is subjected to heat and pressure when it passes through the fixing nip section formed by a heat fixing member and a pressure member of the fixing unit that are in a pressure contact state, and the toner gets fixed on to the recording material due to adhesion.  
         [0006]     During such fixing, there is a phenomenon called blistering that is not desirable for image formation. Blistering is a phenomenon that occurs mainly when coated paper with a low basis weight is used as the recording sheet, and is a phenomenon in which image blemishes appear with the surface of the toner layer becoming textured because, while the coated paper passes through the nip section of the fixing unit, the air or moisture in the toner layer cannot escape to the outside but become bubbles within the toner image, and also because the moisture in the coated paper passes through the coating layer and evaporates.  
         [0007]     While the coated paper is passing through the nip section comprising a heat fixing member which is, for example, a fixing belt, and a pressure member which is, for example, an external pressure roller, the toner image is fixed by melting on top of the recording material, and while it is necessary to take the toner layer to a sufficiently high temperature in order to obtain the necessary glossiness, because of this heating, the recording material and the layer below the toner image that does not contribute to the glossiness are also heated more than is necessary, thereby causing the blistering phenomenon. It has been known that the temperature of the pressure member has a large influence on the temperature of the recording material and of the layer below the toner image, and the generation of blisters is reduced by maintaining the temperature of the pressure member at a low value.  
         [0008]     However, since the external pressure roller is in pressure contact with the fixing belt during image formation, the temperature of the external pressure roller exceeds the permissible temperature due to the heat received by it from the fixing belt through the paper interval or outside the sheet passing region even though it is not heated intentionally, thereby resulting in an environment of generating blisters.  
         [0009]     In Patent Document 1, as a means for preventing the generation of blisters, a configuration has been disclosed in which the set temperatures of the heat fixing roller which is a heat fixing member and of the endless belt shaped pressure member are set at different values, and, based on the respective detected temperature conditions that have been detected using sensors, the heat fixing roller and the pressure member are rotated respectively in a state in which they are separated from each other.  
         [0010]     In the disclosure made in Patent Document 2, in order to prevent wrinkling of the transfer material or disturbances in the fixed image, it has been disclosed to arrange four cooling fans in the axial direction of the pressure roller, these fans are driven according to the size of the sheet, thereby lowering the temperature of the high temperature parts of the pressure roller.  
         [0011]     In the disclosure made in Patent Document 3, as a method of shifting the optimum amount of heating of the fixing roller from the high heating amount to low heating amount, a cooling fan that cools the periphery of the fixing roller, and when the temperature of the fixing roller becomes higher than the set fixing temperature range, based on the temperature detection signal from the temperature detecting means, the control means drives the cooling fan thereby lowering the temperature of the entire fixing roller uniformly.  
         [0012]     Patent Document 1: Unexamined Japanese Patent Application Publication No. Hei 11-194647  
         [0013]     Patent Document 2: Unexamined Japanese Patent Application Publication No. Hei 6-242701  
         [0014]     Patent Document 3: Unexamined Japanese Patent Application Publication No. Hei 10-20707  
         [0015]     In order to prevent the generation of blisters, the method in Patent Document 1 of separating the heat fixing member and pressure member is not desirable because during continuous printing, the problem that the printing has to be interrupted in order to separating these two members, etc.  
         [0016]     Further, in order to cool the external pressure roller, the method of cooling by applying an air blow from outside the roller using a cooling fan as disclosed in Patent Documents 2 or 3 causes heat pollution because the hot air gets dispersed within the equipment after cooling.  
         [0017]     Apart from using a cooling fan, although there is the method of cooling the surface of the roller by making a heat pipe come into contact with the external pressure roller, it is difficult to obtain sufficient cooling effect because a temperature difference is necessary between the surface of the roller and the heat pipe in order to carry out cooling.  
         [0018]     In order to cool the external pressure roller, when detecting the temperature of the external pressure roller and then starting the cooling using these cooling means, there will be a delay before the cooling effect appears due to the thermal capacity of the roller itself and the cooling may not be done in time.  
         [0019]     The purpose of the present invention is to provide a color image forming apparatus provided with a fixing unit that controls the dispersion of heat within the apparatus at the time of cooling the external pressure roller to the minimum level and suppresses the temperature rise of the external pressure roller so that the generation of blisters cannot be observed.  
       SUMMARY OF THE INVENTION  
       [0020]     In order to solve the above problems and also achieve the purpose, one of the configurations according to the present embodiments is an image forming apparatus comprises an image bearing body, an image forming section to form a toner image on the image bearing body, a transfer section to transfer the toner image on a recording material, and a fixing apparatus having a pressure member and a heating member, which fixes the toner image on the recording material transported their between, the fixing apparatus comprises a cooling section to cool the pressure member, and a controlling section to control the cooling section according to an interval between recording materials. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is an outline cross-sectional diagram of the image forming apparatus.  
         [0022]      FIG. 2  is a cross-sectional diagram of the belt fixing unit.  
         [0023]      FIG. 3  is a cross-sectional diagram showing the air flow path.  
         [0024]      FIG. 4  is an outline block diagram of the electrical control system.  
         [0025]      FIG. 5  is a graph showing the relationship between the paper interval ratio and the external pressure roller temperature.  
         [0026]      FIG. 6  is a graph showing the trend of the temperature of the fixing belt and the external pressure roller.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     A preferred embodiment of the present invention is described below. However, the descriptions given here shall not limit the technical scope of the claims or the definitions of terms. Further, the definitive explanations in the preferred embodiment of the present invention below are only indicative of the best mode and shall not limit in any way the meanings of terms or the technical scope of the present invention.  
         [0028]     An image forming apparatus making possible the image formation according to the present invention is described below with reference to  FIG. 1 .  
         [0029]     In  FIG. 1 , the image forming apparatus GS comprises the image forming apparatus main unit GH and the image reading apparatus YS.  
         [0030]     The image forming apparatus main unit GH is a color image processing apparatus of the tandem type and comprises plural sets of image forming sections  10 Y,  10 M,  10 C, and  10 K, a belt shaped intermediate image transfer body  6 , a sheet feeding and conveying means, and a fixing unit  9  to be described later.  
         [0031]     An image reading apparatus YS comprises an automatic document feeder apparatus  201  and a document image scanning exposure unit  203  is placed on top of the image forming apparatus main unit GH. The document D placed on top of the document table of the automatic document feeder apparatus  201  is conveyed by the document conveying means, the image on one side or both sides of the document is scanned and exposed by the optical system of the document image scanning exposure unit  202  and is read in by the line image sensor CCD. At this time, for the document D conveyed from above the document table, the judgment of glossiness of the document image, judgment of monochrome or color of the document image, and the judgment of duplex image are made by the glossiness level detection sensor PKa which is used as a glossiness level selection means.  
         [0032]     The analog signal obtained by photoelectric conversion from the line image sensor CCD is subjected to analog signal processing, A/D conversion, shading correction, image compression, etc., in the image processing section, is stored temporarily in the memory, and then sent as signals to the image writing section (exposure means)  3 Y,  3 M,  3 C, and  3 K.  
         [0033]     The image forming section  10 Y that forms images of yellow (Y) color has a charging unit  2 Y, an exposure unit  3 Y, a development unit  4 Y, and a cleaning unit  8 Y arranged around the periphery of a photoreceptor drum  1 Y which acts as the image bearing body. The image forming section  10 M that forms images of magenta (M) color has a photoreceptor drum  1 M which acts as the image bearing body, a charging unit  2 M, an exposure unit  3 M, a development unit  4 M, and a cleaning unit  8 M. The image forming section  10 C that forms images of cyan (C) color has a photoreceptor drum  1 C which acts as the image bearing body, a charging unit  2 C, an exposure unit  3 C, a development unit  4 C, and a cleaning unit  8 C. The image forming section  10 K that forms images of black (K) color has a photoreceptor drum  1 K which acts as the image bearing body, a charging unit  2 K, an exposure unit  3 K, a development unit  4 K, and a cleaning unit  8 K. The charging unit  2 Y and the exposure unit  3 Y, the charging unit  2 M and the exposure unit  3 M, the charging unit  2 C and the exposure unit  3 C, the charging unit  2 K and the exposure unit  3 K constitute the latent image forming means.  
         [0034]      4 Y,  4 M,  4 C, and  4 K are developing units that store internally two-component developing agent comprise a toner and a carrier of small particle diameters of the colors yellow (Y), magenta (M), cyan (C), and black (K).  
         [0035]     An intermediate image transfer body  6  is wound around a plurality of rollers, and is supported in a rotatable manner.  
         [0036]     The images of each individual color formed by the image forming sections  10 Y,  10 M,  10 C, and  10 K are successively transferred onto the intermediate image transfer body  6  which rotates at a speed of 300 mm/s by the transfer section  7 Y,  7 M,  7 C, and  7 K (primary transfer), thereby forming a synthesized color image. The recording sheet P as the transfer material stored inside the sheet feeding cassette  20  is fed by the sheet feeding means  21 , passes through the sheet feeding rollers  22 A,  22 B,  22 C, and a registration roller  23 , conveyed to the transfer section  7 A and the color image is transferred onto the recording sheet P (secondary transfer) The recording sheet P on which the color image has been transferred is gripped in the pressing nip section N formed in the fixing unit  9  (see  FIG. 2 , not shown in  FIG. 1 ), the color toner image (or the toner image) on the recording sheet P is fixed onto the recording sheet P by applying heat and pressure to it, the recording sheet is then gripped by the sheet discharge rollers  24  on the sheet discharge path side, and is then placed on the sheet discharge tray  25  outside the apparatus.  
         [0037]     During image forming on both sides of the sheet, the recording sheet P with color image (color toner image) formed on one of its surfaces (top surface) and discharged from the fixing unit  9  is branched from the sheet discharge path by the branching means  26 , which respectively constitute the sheet conveying means, passes through the re-circulating sheet path  27 A on the lower side, inverted in the inverting convey path  27 B which is the sheet re-feeding mechanism (ADU mechanism), passes through the sheet re-feeding and conveying section  27 C, and meets again at the sheet feeding roller  22 D. The inversely conveyed (re-circulating inverting conveyed) recording sheet P passes through the timing roller  23  conveyed again to the secondary transfer roller  7 A, where a color image (color toner image) is at once transferred onto the other side (back side) of the recording sheet P. The recording sheet P with color images transferred on to it is fixed by the fixing unit  9 , gripped by the sheet discharge roller  24  on the sheet discharging path side, and is then placed on the sheet discharge tray  25  outside the apparatus.  
         [0038]     On the other hand, after the color image is transferred onto the recording sheet P by the transfer section  7 A, the residual toner on the intermediate transfer body  6  from which the recording sheet P has been separated due to difference in radius of curvature is removed by the cleaning means  8 A  
         [0039]     The belt fixing unit used in the image forming apparatus according to the present invention is explained below referring to  FIG. 2 .  
         [0040]     The fixing unit  9  comprises a fixing belt  91  which is a metallic base or a heat resistant plastic base and silicone rubber formed in the shape of a belt, the supporting pressure roller  92  that supports and conveys this fixing belt and carries out fixing by applying pressure and heat while gripping the transfer material and the fixing belt  91 , and an external pressure roller  93  opposing this, and a supporting heating roller  94  that has inside it a heater  94   a  and that supports and conveys the fixing belt  91 . In the belt fixing unit  9  of the above configuration, the pressure nip section N is formed by the supporting pressure roller  92  and the external pressure roller  93 .  
         [0041]     The supporting pressure roller  92  is a soft roller with an external diameter of 40 mm having a rotating shaft  921  made of a cylindrical metal pipe of a wall thickness of 2-5 mm or of a metal shaft, and an elastic layer  922  of 7 mm thick silicone rubber is adhered on to the rotating shaft  921  on the outer periphery.  
         [0042]     The external pressure roller  93  is a soft roller with an external diameter of 50 mm having a hollow rotating shaft  931  with an internal diameter of 38 mm and made of a cylindrical aluminum pipe of a wall thickness of 3 mm, an elastic layer  932  of 2 mm thick silicone rubber is adhered on the outer periphery of the rotating shaft  931 , and the outside of which is provided a PFA (Perfluoroalkoxyethylene) tube  933  as the separating layer.  
         [0043]     Since a softer material has been used for the elastic layer  922  of the supporting pressure roller  92  compared to the elastic layer  932  of the external pressure roller  93  and also since the elastic layer is thick, in the pressing nip section N the supporting pressure roller  92  gets deformed mainly thereby maintaining the pressure contact state. In the preferred embodiment, a 14 mm nip section is being formed by applying a weight of 900N. In addition, during the warming up period when pre-heating is carried out, the external pressure roller  93  is in contact with the supporting pressure roller  92  via the fixing belt  91  with a pressing force of 350N. Further, the external pressure roller  93  maintains the pressing state during continuous printing, and moves to the separated state once the printing operation is completed.  
         [0044]     In the present preferred embodiment, air flow due to an ON/OFF controlled fan motor  951  is passed inside the external pressure roller  93  via a duct  952 .  FIG. 3  is a cross-sectional diagram showing the air flow path.  
         [0045]     Due to the drive of the fan motor  951 , the air outside the apparatus is sucked from the air suction inlet  952   a  which is one end of the duct  952  and is exhausted from the air exhaust outlet  952   b . Since the external pressure roller  93  moves in contrast with the duct  952  which is in a fixed position, there is a air flow preventing member  935  at the end of the external pressure roller  93  opposite the air exhaust outlet  952   b , all the air exhausted from the air exhaust outlet  952   b  passes through the hollow external pressure roller  93  thereby cooling it, and in the present preferred embodiment, the drive rotation of the fan motor  951  is switched between the two air speed levels of 12 m/s and 6 m/s of the air flowing inside the external pressure roller  93 . The warm air coming out of the external pressure roller  93  passes through the duct  953  for air exhaust, and is discharged to outside the apparatus along with other exhaust air by the axial flow fan  954 . Further, since the air flow path is narrow for the air flow with the fan motor  951 , a sirocco fan, which has a high static pressure and is suitable for applications with large pressure losses, is used desirably, however, it is not necessary to limit to such a fan.  
         [0046]     The fixing belt  91  trained about the supporting pressure roller  92  is an 80 mm diameter endless belt having a 70 μm thick polyimide base on the outside of which is provided a 200 μm silicone rubber layer and a 30 μm PTFE coating layer. The fixing belt  91  is heated by thermal conduction transmitted via the heating roller from the heater  94   a  built inside the supporting heating roller  94  around which the belt is passed.  
         [0047]     The supporting heating roller  94  is a 50 mm external diameter roller which is an aluminum core metal of wall thickness 2 mm on the outer periphery of which is provided a fluorocarbon resin layer, and slide guards are provided at the two ends of the roller thereby restricting the position of the fixing belt  91  so that it does not slide in the axial direction.  
         [0048]      FIG. 4  shows an outline block diagram of the electrical control system. In this Fig.,  110  is a CPU that carries out computer control processing to which are connected a RAM  111  and a ROM  112 . The ROM  112  stores basic data for computation, the simplex image forming mode program, the duplex image forming mode program, the external pressure roller cooling program of the present invention, etc, and the CPU  110  is connected to external devices via the interface  120 .  
         [0049]     The image forming section  121  such as the charging unit  2 , the exposure unit  3 , the developing unit  4 , etc., the fixing unit  9 , the recording sheet transporting section  122  that carries out feeding, conveying, inverted conveying, sheet discharge, etc., of the recording sheet are connected to the interface  120 .  
         [0050]     Further, in the image forming apparatus shown in  FIG. 1 , an operation/display section  30  has been provided which has the START button for instructing the starting of the print operations, the size selection button for selecting the size of the recording sheets used, the ten-keys for specifying the number of recording sheets to be printed, the image density selection buttons for selecting the image density, and also the singe side image or duplex image printing, and also the magnification correction and no magnification correction selection buttons during duplex image printing.  
         [0051]     When the user selects the simplex image printing and then pushes ON the START button in the operation/display section  30 , the CPU  110  calls the simplex image forming mode program from the ROM  112 , and carries out control of image forming according to the called program. In other words, the CPU  110  controls the recording sheet transporting section  122  so that it issues the recording sheet P from the corresponding sheet feeding cassette  20 , controls the image forming section  121  so that it transfers the toner image formed on the intermediate image transfer body  6  on to the recording sheet P, and carries out control so that the toner image is fixed by passing the recording sheet P carrying a toner image on one of its sides through the fixing unit  9 , and then the recording sheet P is discharged to outside the apparatus by passing through the sheet discharge path.  
         [0052]     Further, when the user selects the duplex image printing and then pushes ON the START button in the operation/display section  30 , the CPU  110  calls the duplex image forming mode program from the ROM  112 , and carries out control of image forming according to the called program. In other words, the CPU  110  controls the recording sheet transporting section  122  so that it issues the recording sheet P from the corresponding sheet feeding cassette  20 , controls the image forming section  121  so that it transfers the toner image formed on the intermediate image transfer body  6  on to one surface (the top surface) of the recording sheet P, and carries out control so that the toner image is fixed by passing the recording sheet P carrying a toner image on one of its sides (the top surface) through the fixing unit  9 , then conveys the recording sheet via the inverting conveying path again to the transfer section  7 A, controls the image forming section  121  so that it transfers the toner image formed on the intermediate image transfer body  6  on to the other surface (the back side) of the recording sheet P, and carries out control so that the toner image is fixed by passing the recording sheet P having a toner image transferred on to it through the fixing unit  9 , and then the recording sheet P having images on both sides is discharged to outside the apparatus by passing through the sheet discharge path.  
         [0053]     In the present preferred embodiment, it is possible to select in the duplex image forming mode program any one of the two modes with magnification correction or without magnification correction in top and backside images.  
         [0054]     When high image quality is required, the mode with magnification correction is provided in order to prevent a difference being present in the image magnification ratios of the top and back sides because the recording sheet shrinks due to fixing during duplex image formation, and when the magnification correction mode has been selected, changes are made in the magnification ratios of unfixed images during image formation on the top and back sides. Since the polygon rotational speed is changed while changing the magnification ratio, in order to acquire time for changing the rotational speed, the control is carried out so that the output is made at every three sheets while skipping the image formation and fixing of one sheet in the mode with magnification correction as compared to carrying out top and back side fixing for every four sheets normally in the present preferred embodiment.  
         [0055]     On the other hand, if high speed printing operations are required during duplex image formation, the mode without magnification correction is selected, and control is carried out so that top and back side fixing is carried out at every four sheets.  
         [0056]     In the present invention, in parallel with the simplex image formation or duplex image formation mode program, the external pressure roller cooling program stored in the ROM  112  is called, and the CPU  110  carries out control of the cooling operations of the external pressure roller  93  by driving the fan motor  951  based on the information input from the operation/display section  30 .  
         [0057]     In the preferred embodiment described next, the external pressure roller cooling program is a program that controls the operation of the fan motor  951 , which is a cooling means, based on the sheet interval ratio at the time of image formation.  
         [0058]     When the temperature of the external pressure roller is predicted to rise up to the temperature at which blisters occur at the sheet interval ratio and number of sheets to be printed specified in the image output signal from the operation/display section  30 , the air flow due to the fan motor  951  is started before the temperature of the external pressure roller rises up to the temperature at which blisters occur. The rise in the temperature of the external pressure roller is determined by the basis weight of the sheet and the sheet interval ratio, and should be determined by experiment for each fixing system.  
         [0059]     The sheet interval ratio is defined as the ratio of the sheet interval (the space between one print sheet and the next print sheet) to the distance traveled by the fixing member (in this case, the fixing belt  91 ). When A4 sized (210 mm) sheets are fixed at 90 mm sheet interval, that is, when fixed at 60 ppm (prints per minutes) at a speed of 300 mm/s, the sheet interval ratio is 90/(210+90)=0.3. And, if the productivity is made ¾ in order to match the top and back sides (magnification ratio correction) during duplex image formation as in the manner described above, the sheet interval ratio becomes (90×4+210)/{(210+90)×4}=0.48.  FIG. 5  is a graph showing an example of the relationship between the paper interval ratio and the saturation external pressure roller temperature.  
         [0060]     From the above figure, it can be seen that there is a trend for the external pressure roller saturation temperature to rise as the sheet interval ratio becomes larger. Further, it indicates that there is a trend of the external pressure roller saturation temperature to decrease as the basis weight increases in the coated sheet.  
         [0061]     The graph shown in  FIG. 6  gives the temperature trend C 6 B of the fixing belt  91  and the temperature trend C 6 A of the external pressure roller  93  when 69 g/m 2  sheets are fixed at a rate of 45 ppm. The temperature of the external pressure roller  93  slowly increases by receiving heat from the belt between sheets due to pressure contact with the high temperature fixing belt  91 . After 250 s from the start of sheet feeding, that is, after about 200 sheets are printed, the temperature of the external pressure roller  93  reaches 140° C., which is a level at which blisters appear significantly with intolerable level.  
         [0062]     At sheet interval ratios and sheet basis weights at which the temperature of the external pressure roller  93  exceeds the permissible temperature in this manner, the fan motor  951  for cooling is driven, thereby suppressing the temperature rise of the external pressure roller  93 . When air at room temperature is passed at an air flow speed of 12 m/s by the fan motor  951  inside the rotating shaft  922  which is the metal core, the saturation external pressure roller temperature under the same conditions was 115° C., and hence it was possible to suppress the temperature to a level at which the blistering was permissible. Of course, when wanting to suppress blistering more completely, it is also possible to operate at a still lower estimated temperature rise.  
         [0063]     In actual control, even without calculating the sheet interval ratio, it is possible to determine the operation of the fan motor  951  based on a table of sheet size and productivity etc. An operation table of sheet size versus cooling fan operation is shown below as an example.  
                                   TABLE 1                                           Duplex print   Duplex print                   (without   (with           Sheet   Simplex   magnification   magnification           size   print   ratio correction)   ratio correction)                           B6   L   H   H           B5   L   H   H           B4   L   L   H           A6   L   L   H           A5   OFF   L   H           A4   OFF   OFF   L           A3   OFF   OFF   OFF                      
 
         [0064]     The CPU  110  drives the fan motor  951  in any one of the states of H (motor drive to get an air flow speed of 12 m/s), L (motor drive to get an air flow speed of 6 m/s), and OFF according to the sheet size at which the printing is to be made input from the operation/display section  30  and depending on which print mode is being used among simplex, duplex (without magnification ratio correction), duplex (with magnification ratio correction).  
         [0065]     Further, it is desirable to add also in this table the information from the temperature sensor or the humidity sensor installed in the image forming apparatus, and to carry out control so that the external pressure roller temperature is still lower under conditions in which blisters are likely to be generated.  
         [0066]     In the present preferred embodiment, in order to suppress to a minimum the discharge of warm air to outside the apparatus, the fan is not driven if the temperature rise of the external pressure roller is estimated to be at a level at which blistering is not noticeable. Since, by not driving the fan it is possible to reduce the heater power and the fan power consumptions, it is desirable not to drive the fan when not necessary from the point of view of energy saving. It is also possible to drive the fan at all times when image quality is given priority and when using sheets with the basis weight being such that blisters are easily generated (for example, sheets of 100 g/m 2  or less).  
         [0067]     The fan is stopped when sheet conveying is finished. Since the temperature inside the external pressure roller is lower, even if the fan is stopped immediately after the sheet conveying is finished, there is no possibility of the sudden temperature rise as in the case of the fixing roller which has a built-in heater.  
         [0068]     The cooling air is injected from the opening section at the ends of the external pressure roller rotating shaft  931  and is exhausted from the outlet section on the other side. Compared to the case of cooling the roller from the outside, forming the air guidance path is easier and it is possible to suppress dispersion of heat to the interior of the apparatus.  
         [0069]     Further, by maintaining the external pressure roller temperature low, the temperature difference occurring at the boundary between the area of contact of the belt with the external pressure roller and the area of contact with the sheet remains up to one revolution of the belt, and have the effect of reducing the defects of creating steps in the glossiness.  
         [0070]     By having this type of configuration and control, it is possible to provide an image forming apparatus that can carry out high quality image formation with suppressed generation Of blisters and without defects caused by the temperature rise inside the apparatus. Further the fixing unit may be provided with temperature detector  96  to detect the temperature of the external pressure roller, and the cooling means may be configured to be controlled based on the temperature of the external pressure roller.  
         [0071]     In the preferred embodiment described next, the external pressure roller cooling program is one that carries out control of the operation of the fan motor  951 , which is the cooling means, based on the basis weight of the recording sheet during image formation.  
         [0072]     The generation of blisters is decided by the basis weight of the recording sheet and the temperature of the external pressure roller. The status of generation of blisters in the present preferred embodiment is shown in the table below.  
                                                                               TABLE 2                                       External pressure           roller temperature (° C.)                100   120   140   160   180                             80 g/m 2  sheet   A   B   C   C   C           100 g/m 2  sheet   A   A   B   C   C           128 g/m 2  sheet   A   A   A   B   C           158 g/m 2  sheet   A   A   A   A   A                      
 
         [0073]     In the above table, C indicates that the rate of generation of blisters is high, B indicates that blisters are generated occasionally during image formation, and A indicates that good fixed images can be obtained without the generation of blisters.  
         [0074]     The sheets are in contact with the external pressure roller  93  during sheet conveying, if the amount of heat supplied from the belt through the contact between the fixing belt  91  and the external pressure roller  93  during the intervals of sheets is more than the amount of heat taken away from the external pressure roller  93 , the temperature of the external pressure roller increases gradually. When a thin coated sheet of 80 g/m 2  is used with a speed of 300 mm/s at a productivity rate of 65 ppm, the temperature of the external pressure roller stays at around 120-130° C. During the operation of carrying out magnification ratio correction in the duplex image forming mode, the productivity becomes 48 ppm and the sheet interval becomes double, the external pressure roller temperature becomes 150° C. when using 80 g/m 2  sheets, and increases up to 140° C. even with 100 g/m 2  sheets.  
         [0075]     Further, although the above table is for an ambient temperature of 25° C. and an ambient humidity of 40% RH, at still higher temperatures and humidity, if the moisture content of the sheet becomes higher, the temperature of generation of blisters becomes lower.  
         [0076]     Also in the present preferred embodiment, the control operation of the fan motor  951  can be determined based on the table of the basis weight of the sheets and the productivity, etc. The following is an example showing a table of basis weight of sheets and cooling fan operation.  
                                                         TABLE 3                                           Duplex print   Duplex print           Basis       (magnification   (magnification           weight       correction not   correction           (g/m 2 )   Simplex print   present)   present)                                        80   L   H   H           100   OFF   L   H           128   OFF   OFF   L           158   OFF   OFF   OFF                      
 
         [0077]     The CPU  110  drives the fan motor  951  in any one of the states of H, L, and OFF from the above table according to the basis weight of sheets with which the printing is to be made input from the operation/display section  30  and depending on which print mode is being used among simplex, duplex (without magnification ratio correction), duplex (with magnification ratio correction).  
         [0078]     By having this type of configuration and control, it is possible to provide an image forming apparatus that can carry out high quality image formation with suppressed generation of blisters and without defects caused due to temperature rise inside the apparatus.  
         [0079]     In the above explanations, cooling the external pressure roller  93  as necessary was described as a means for preventing blisters. However, it is not desirable to cool the external pressure roller  93  unconditionally.  
         [0080]     Since the problem of increased curling of sheets occurs if the temperature of the external pressure roller becomes too low, the heating is maintained up to a temperature of about 100° C. Since the cooling performance is lost if a heater is placed inside the external pressure roller  93 , a heater was not provided in the present preferred embodiment. Instead, the fixing belt  91  and the external pressure roller  93  are rotated while pressing against each other, and the external pressure roller  93  is heated through the belt. When raising the temperature of the external pressure roller immediately after switching ON the power supply or after a waiting period, the fan motor  951  is kept stopped, current is supplied through the heater of the heating roller, and the external pressure roller  93  is rotated in pressure contact with the belt.  
         [0081]     If there is any margin in the cooling air flow path or in the capacity of the air blowing means, it is also possible to incorporate a heater inside the external pressure roller  93  for heating it quickly and without placing a load on the belt.  
         [0082]     According to the above embodiment, by carrying out the control of cooling the external pressure member which is the pressing member so as to carry out cooling operation based on the sheet interval ratio at the time of image formation, the cooling means is controlled to be operated in an positive manner under conditions in which temperature rise of the external pressure member is likely to occur, thereby preventing effectively the rise of the external pressure roller temperature up to the temperature at which blisters can be generated easily, and hence it is possible to obtain satisfactorily fixed images without the generation of blisters.  
         [0083]     According to the above embodiment, by carrying out control of the cooling means that carries out cooling of the external pressure member so as to carry out cooling operation based on the basis weight of recording sheets at the time of image formation, the cooling means is controlled to be operated in an positive manner to the recording sheets, the basis weight of which can generate blisters easily, thereby preventing effectively the rise of the external pressure roller temperature up to the temperature at which blisters can be generated easily in each recording sheet, and hence it is possible to obtain satisfactorily fixed images without the generation of blisters.  
         [0084]     According to the above embodiment, cool air from outside the apparatus is supplied with an ensured air flow rate into the metal core of the hollow external pressure roller which is linked to a fan via a duct, it is possible to obtain satisfactorily fixed images without the generation of blisters, because the cooling operation is carried out satisfactorily without the warm air after cooling getting dispersed inside the apparatus and causing thermal contamination.