Patent Publication Number: US-8983338-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-267245, filed Dec. 6, 2011, and Japanese Patent Application No. 2012-150240, filed Jul. 4, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus, which is provided with a mechanism for preventing trouble from happening, in which the users get their hands burned by a hot heat fixing unit, while taking out the heat fixing unit from the apparatus for maintenance purpose. 
     2. Description of the Related Art 
     Image forming apparatuses are widely used, which employ an electro-photographic toner system to form a toner image on an image-supporting member, and transfer the toner image onto a paper, and then fix the transferred toner image on the paper using a fixing unit. 
     These image forming apparatuses are provided with plural detachable internal units such as a belt unit, an image forming unit, and an image fixing unit. In the maintenance operation of these units, users take out the internal units from the apparatus body to repair or replace them. 
     For example, during the printing operation of the image forming apparatus, the temperature of image fixing unit is kept at around 110 to 180 degrees Centigrade. When a jammed paper happens in the image fixing unit and the user tries to take out the image fixing unit from the apparatus body to remove the jammed paper, the user&#39;s carelessness can invite an accident of getting burned. 
     To prevent the above accident from happening when the user takes out the image fixing unit from the apparatus body, an image fixing unit of a printer is proposed by Japanese Unexamined Patent Publication Hei 06-274054, in which a character string of “CAUTION HOT” is printed on the peripheries of a pressure roller and a heat roller consisting the image fixing unit to draw the user&#39;s attention. 
     In general, the jammed papers in the image fixing unit are roughly classified into two groups: one group including the jammed papers wrapping around either of the pressure roller and heat fixing roller, with a portion of the rear edge of the paper left outside the fixing unit, and other group including the jammed papers completely wrapping around either of the pressure roller and heat fixing roller with no portion left outside the fixing unit and the jammed papers fold up into concertinas and staying within the fixing unit. 
     In the case of the jammed papers wrapping around the roller with the rear edge of the paper left outside the fixing unit, in many cases, the user can remove the jammed paper from the fixing unit by pulling out the rear edge of the paper. But in the case of the jammed papers completely wrapping around the roller with no portion left outside the fixing unit and the jammed papers fold up into concertinas and staying within the fixing unit, it is impossible to remove the jammed paper from the fixing unit with the fixing unit installed within the apparatus. 
     In the case of the jammed papers, the whole size of which completely stays within the fixing unit, the user is required to take out the fixing unit from the image forming apparatus and to open a protection cover with careful attention not to get burned to remove the jammed paper from the fixing unit. 
     As disclosed in Japanese Unexamined Patent Publication Hei 06-274054, the character string of “CAUTION HOT” is printed on the peripheries of the pressure roller and the heat roller, but these rollers are disposed within a housing of the image fixing unit. Therefore, even though the user takes out the image fixing unit from the printer body, there will be no chance for the user to read the caution of “CAUTION HOT”, when the protection cover is not open. 
     Further, in the case of the paper completely wrapping around with no edge left outside the fixing unit, even if the protection cover is opened, the caution of “CAUTION HOT” is covered with the wrapping paper, which will prevent the user from reading the caution of “CAUTION HOT”. In this case, the character string of “CAUTION HOT” printed on the rockers will be of no effect to draw the user&#39;s attention, when the hot fixing unit is taken out of the printer body 
     From the viewpoint of safety first, that is, protecting the users from getting burned, the fixing unit proposed by Japanese Unexamined Patent Publication Hei06-274054 is not a device safe enough, and a problem is still left that further measures for safety are required. 
     SUMMARY OF THE INVENTION 
     The present invention is to solve the conventional problem mentioned above, and provides an image forming apparatus that is provided with a mechanism, which protects the users from getting burned when they take out the fixing unit from the apparatus body for maintenance. 
     According to one aspect of the present invention, there is provided an image forming apparatus, which comprises an apparatus body having a frame, a detachable fixing unit having fixing paired-rollers, and installed in the apparatus body, a heating/controlling unit for heating and controlling at least one roller of the fixing paired-rollers of the detachable fixing unit, a driving source unit for driving the one roller of the fixing paired-rollers heated by the heating/controlling unit, a temperature detecting unit for detecting a temperature of the detachable fixing unit, and an adjustment mechanism for preventing the detachable fixing unit from being taken out from the apparatus body, when the temperature detecting unit determines that the temperature of the detachable fixing unit exceeds a predetermined value, and for allowing the detachable fixing unit to be taken out from the apparatus body, when the temperature detecting unit determines that the temperature of the detachable fixing unit does not exceed the predetermined value. 
     As described above, the present invention provides the image forming apparatus provided with the mechanism, which protects the users from getting burned when they take out the fixing unit from the apparatus body for a maintenance purpose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention will be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  is a cross-sectional view showing an internal construction of a full color image-forming apparatus (printer) according to embodiments of the present invention. 
         FIG. 2  is a perspective view showing an appearance of the printer according to the embodiments of the present invention. 
         FIG. 3A  is a view showing the printer with an access member half-open. 
         FIG. 3B  is a view schematically showing an inside of the printer body with the access member open. 
         FIG. 4A  to  FIG. 4D  are views for explaining a series of operations of taking out a fixing unit from the printer body and removing a jammed paper from the fixing unit. 
         FIG. 5A  is a schematic cross-sectional view showing the fixing unit of the printer in a printing operation. 
         FIG. 5B  is an enlarged schematic view showing the main portion of the fixing unit. 
         FIG. 5C  is a detailed cross-sectional view of the fixing unit taken along the line A 1 -A 1  of  FIG. 5B . 
         FIG. 6A  is a view showing the cam shaft of  FIG. 5A , which rotates 180 degrees from a position shown in  FIG. 5C . 
         FIG. 6B  is a view showing the cam shaft of  FIG. 5B , which rotates 180 degrees from the position shown in  FIG. 5C . 
         FIG. 6C  is a view showing the cam shaft of  FIG. 5C , which rotates 180 degrees from the position shown in  FIG. 5C . 
         FIG. 7A  is a front view showing the fixing unit and a driving system for driving its mechanical locking mechanism. 
         FIG. 7B  is a side view showing the fixing unit and the driving system for driving its mechanical locking mechanism. 
         FIG. 8A  is a view showing the fixing unit, which is in a state to be engaged with a driving system of the fixing unit. 
         FIG. 8B  is a view showing the fixing unit, which is completely engaged with the driving system of the fixing unit. 
         FIG. 9A  is a view showing the fixing unit, which is installed to the installing portion of the printer body. 
         FIG. 9B  is a view showing the fixing unit, which is installed and locked to the installing portion of the printer body. 
         FIG. 10  is a block diagram of an electric unit of the printer, including a controlling unit. 
         FIG. 11  is a flow chart of a normal printing process performed by the printer according to the present embodiment of the invention. 
         FIG. 12  is a flow chart of a process performed in case of a jammed paper. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a cross-sectional view showing an internal construction of a full color image-forming apparatus (hereinafter, referred to as a “printer” or “present apparatus”) according to the embodiment 1 of the present invention. 
     The full color image-forming apparatus  1  has a tandem configuration, as shown in  FIG. 1  and is a printer of a secondary electro-photographic transfer system. The printer  1  comprises an image forming unit  2 , a transfer belt unit  3 , a toner feeding unit  4 , a paper feeding unit  5 , a belt-type heat fixing unit  6  (hereinafter, referred to as a “fixing unit”), and a double-sided printing conveyance unit  7 . 
     The image forming unit  2  is a multistage developing device  9 , which consists of four developing devices  9  ( 9   k ,  9   c ,  9   m , and  9   y ). These developing devices  9  ( 9   k ,  9   c ,  9   m , and  9   y ) align right to left, as seen in  FIG. 1 , so as to contact with an under running surface  8   a  of the transfer belt  8  of the transfer belt unit  3 . The image forming unit  2  is held to a frame of the printer body  1  so as to move from a position (printing position) for a printing operation shown in  FIG. 1  downward to a position for maintenance. 
     Three developing devices  9   c ,  9   m , and  9   y  arranged upstream (left side in  FIG. 1 ) among the four devices  9  are provided for forming three mono-color images, that is, images of subtractive primary three colors, Cyan (C) toner, Magenta (M) toner, and Yellow (Y) toner, respectively. The developing device  9   k  is provided for forming a monochrome image of black (K) toner, used for characters and dark parts in an image. 
     All the developing devices  9  ( 9   k ,  9   c ,  9   m , and  9   y ) have the same structure except the toner used therein. Using the example of the developing device  9   y  for the yellow (Y) toner, the structures of these developing devices will be described hereinafter. 
     The developing device  9  is provided with a photoreceptor drum  10  at its top. The photoreceptor drum  10  has an outer periphery, which is made of an organic photoconductive member. There are arranged a cleaner  11 , a charge roller  12 , an optical writing head  13 , a developer  14 , and a developing roller  15  around the periphery of the photoreceptor drum  10 . 
     Further, the developer  14  is provided with an external housing  16 , an internal partition wall  17 , the developing roller  15 , a first mixing/feeding screw  18 , and a second mixing/feeding screw  19 . The first and second mixing/feeding screws  18 ,  19  each have a screw shaft and a rotary fin integrally fixed to the shaft (these are not shown). 
     Either of the toners, Black (K), Cyan (C), Magenta (M), and Yellow (Y) indicated by K, C, M, and Y in  FIG. 1  is fed to the developer  14  from reserve tanks  27  ( 27   k ,  27   c ,  27   m , and  27   y ) of the toner feeding unit  4 . 
     The transfer belt unit  3  is provided with an endless transfer belt  8 , a driving roller  21  and a driven roller  22 . The endless transfer belt  8  is extended between the driving roller  21  and the driven roller  22  substantially in the horizontal direction as viewed in  FIG. 1  and is driven in the counter clockwise direction, as indicated by an arrow “a” in  FIG. 1 . 
     The transfer belt  8  has a primary transfer roller  20 , which is integrally provided together with the unit. The primary transfer roller  20  is resiliently pressed onto the photoreceptor drum  10  through the transfer belt  8  to transfer a toner image onto the surface of the transfer belt  8  (primary transfer). Further, the transfer belt  8  carries the toner image to a secondary transferring unit  23  to transfer the toner image onto a paper (secondary transfer). 
     A belt cleaner  24  is set to the transfer belt  8 . The belt cleaner  24  has a cleaning blade  25 , which contacts the surface of the transfer belt  8  driven by the driving roller  21 . A detachable waste-toner recovery container  26  is mounted beneath the belt cleaner  24 . 
     Using the cleaning blade  25 , the belt cleaner  24  serves to rub out the waste toner staying on the surface of the transfer belt  8 , and sends the rubbed out toner into the waste-toner recovery container  26  with its feeding screw. 
     The toner feeding unit  4  consists of four reserve tanks  27  ( 27   k ,  27   c ,  27   m , and  27   y ) and four detachable toner cartridges  28  ( 28   k ,  28   c ,  28   m , and  28   y ) for supplement toner. The reserve tanks  27   k ,  27   c ,  27   m , and  27   y  are disposed on the upper side of the upper running portion of the transfer belt  8 , and further the detachable toner cartridges  28   k ,  28   c ,  28   m , and  28   y  are disposed respectively on the reserve tanks  27  ( 27   k ,  27   c ,  27   m , and  27   y ). 
     The detachable toner cartridges  28   k ,  28   c ,  28   m , and  28   y  contain the toners, Black (K), Cyan (C), Magenta (M), and Yellow (Y), respectively, and the reserve tanks  27  ( 27   k ,  27   c ,  27   m , and  27   y ) are refilled with the toners from the toner cartridges  28   k ,  28   c ,  28   m , and  28   y , respectively. 
     The four reserve tanks  27  are connected respectively to the developers  14  of the developing device  9  through toner feeding paths (not shown). 
     The toner feeding unit  4  is held to the frame of the printer body  1  so as to move from the printing position shown in  FIG. 1  upward to a position for maintenance. 
     On the left side of the toner feeding unit  4 , two electric units  30 ,  30  are disposed around the belt cleaner  24  and the driving roller  21 . The electric unit  30  involves circuit boards with an electronic controlling unit consisting of plural electronic parts. 
     The paper feeding unit  5  consists of two paper feeding cassettes  29  ( 29   a ,  29   b ), one being piled on top of another, as shown in  FIG. 1 . In the vicinity of the paper feeding openings (right side) of the two paper feeding cassettes  29   a ,  29   b , there are provided paper take-up rollers  31 ,  31 , paper-sending rollers  32 ,  32 , paper-handling rollers  33 ,  33 , and stand-by conveying paired-rollers  34 ,  34 . 
     In the paper conveying direction (in the vertical direction as viewed in  FIG. 1 ), in which a paper is conveyed by the stand-by conveying paired-rollers  34 , there is provided a secondary transfer roller  35 , which is resiliently pressed against the driven roller  22  with the transfer belt  8  held between them. The transfer belt  8 , driven roller  22  and the secondary transfer roller  35  operate together to function as the secondary transferring unit  23  for transferring a toner image onto the paper. 
     The fixing unit  6  is disposed downstream (upper side of  FIG. 1 ) of the secondary transferring unit  23 . Further downstream of the fixing unit are arranged discharging paired-rollers  36  for taking up the paper from the fixing section  15  and paper ejecting paired-rollers  38  for ejecting the paper onto a paper ejecting tray  37  formed on the top surface of the printing apparatus  1 . 
     The double-sided printing conveyance unit  7  has an outer surface (a right side cover as viewed in  FIG. 1 ), which serves as an access member of the printing apparatus  1 . 
     The double-side printing conveyance unit  13  provides a return path including a starting return path  39   a , an intermediate return path  39   b , and a terminal return path  39   c . The starting return path  39   a  diverges from the paper ejecting paired-rollers  38  to the right (as viewed in  FIG. 1 ), and the intermediate return path  39   b  follows the starting return path  39   a  and runs downward to the terminal return path  39   c , and the terminal return path  39   c  turns to the left (as viewed in  FIG. 1 ) to finally make the paper turn over. 
     In mid-course of the return path  39 , there are provided five sets of return paired-rollers  41  ( 41   a ,  41   b ,  41   c ,  41   d  and  41   e ). An outlet of the terminal return path  39   c  leads to a paper-conveying path to the stand-by conveying paired-rollers  34  corresponding to the lower paper feeding cassette  29   b  of the paper feeding unit  5 . 
       FIG. 2  is a perspective view showing an appearance of the printer  1 . In  FIG. 2 , like component parts as those in  FIG. 1  are designated by like reference numerals. 
     As shown in  FIG. 2 , the printer  1  is provided with a front door (an opening-and-closing door)  42  in its front, and an access member  44  with a hold  43  on its right side. The double-sided printing conveyance unit  7  shown in  FIG. 1  is integrated with the access member  44 . 
     The printer  1  does not employ a system, which transfers the toner image directly onto the paper, but a system, which transfers the toner image by means of the transfer belt  8  to the paper, which is conveyed in the vertical direction to the secondary transferring unit by the stand-by conveying paired-rollers  34 . 
     Therefore, since troubles such as paper jams do not arise in a kit-disposed portion, the printer  1  is constructed to allow a user to replace consumables such as kits concentrating to the left side in  FIG. 1  with the front door  42  open. With the front door  42  (shown in  FIG. 2 ) open, the user can replace the kits by pulling them towards the front door  42 . 
     The printer  1  employs the system, which transfers the toner image through the transfer belt  8  onto the paper, which is conveyed in the vertical direction to the secondary transferring unit, and further conveys the paper in the vertical direction to fix the transferred toner image on the paper. 
     Therefore, in the case where a maintenance work is performed to recover the troubles such paper jams arising along the paper conveyance path, it is simply required to keep the access member  44  open, as shown in  FIG. 3A . 
     But, even in the case where the paper jam arises along the paper conveyance path, when such paper jam arises within the fixing unit  6  and/or the whole jammed paper is involved within the fixing unit  6 , in order to solve the trouble of the jammed paper, the fixing unit  6  is required to be pulled out from the printer body  1  with the access member  44  and the front door  42  open. 
       FIG. 3A  is a view showing the printer  1  with the access member  44  half-open. At the time when the maintenance is performed when the paper jam arises within the fixing unit  6 , the access member on the right side of the printer body  1  is open as shown in  FIG. 3A .  FIG. 3B  is a view schematically showing the inside of the printer body  1  with the access member  44  open. 
     As shown in  FIG. 3A , when the access member  44  of the printer  1  is open in the direction indicated by an arrow “f”, the inside of the printer body  1  can be seen from an opening  45 . In the inside of the printer body  1 , the belt-type heat fixing unit  6  is seen at the upper side and a rear edge of a jammed paper  46  coming down from the fixing unit  6  can be seen as shown in  FIG. 3B . 
     In this case, the user can remove the jammed paper  46  by pulling down the rear edge of the jammed paper  46 . In the present embodiment, as will be described later in detail, since a resilient pressure applied to Nip portion of the fixing paired-rollers is automatically released when the fixing unit  6  is unlocked from its operating position, and therefore, the jammed paper  46  can be easily removed downwards. 
     It is also possible to provide a lever on a driving roller of the fixing paired-rollers to rotate the driving roller in the forward direction, and the user uses the lever to rotate the fixing paired-rollers, thereby making the jammed paper  46  pass through of Nip portion of the fixing paired-rollers and removing the jammed paper  46  upwards. 
     As described above, generally in many cases, it is possible to remove the jammed paper  46  involved in the fixing unit  6  from the printer body  1  with the fixing unit  6  mounted within the printer body  1  without taking out the fixing unit  6  from the printer body  1 . 
     Some states of the jammed paper can make it hard to remove the jammed paper  46  as in the above mentioned manners. For instance, in the case where an extremely thin paper is used, the thin paper wraps around one of the fixing paired-rollers, and the whole paper can stay within the fixing unit  6 . 
     Further, in the case where a high density image is printed to the edge of the paper, the toner melting to the glass-transition point in the fixing unit  6  sticks onto the heat roller, preventing the paper from separating from the heat roller and causing the jammed paper. 
     Furthermore, the front edge of the paper is caught by something in the fixing unit  6  and the remaining portion of the paper sent into the fixing unit  6  can be fold up into concertinas. In this case, since whole paper stays in the fixing unit  6 , the jammed paper  46  cannot be removed simply by opening the access member  44 . 
     In these cases, it will be necessary for calling for assistance of a serviceman. But the users have to wait for several hours before the serviceman comes to his or her office. To solve the user&#39;s inconvenience, some printer is constructed such that the user is allowed to take out the fixing unit  6  itself from the printer body  1  and to open a protection cover of the fixing unit  6  to remove the jammed paper from the fixing unit  6 . 
       FIG. 4A  to  FIG. 4D  are views for explaining a series of operations of taking out the fixing unit  6  from the printer body  1  and removing the jammed paper  46  from the fixing unit  6 . In  FIG. 4A  to  FIG. 4D , like component parts as those in  FIG. 1  to  FIG. 3A  and  FIG. 3B  are designated by like reference numerals. 
       FIG. 4A  is a view showing the printer body  1  with the front door  42  open. As shown in  FIG. 4A , the front door  42  is held substantially in the horizontal direction. On the upper side of the transfer belt unit  3 , four replaceable toner cartridges  28   k ,  28   c ,  28   m , and  28   y  are disposed. Further, the fixing unit  6  is disposed on the right side of the cartridge  28   k.    
     On the lower side of the transfer belt unit  3 , four developing devices  9   k ,  9   c ,  9   m , and  9   y  are disposed, which are detachable for maintenance. A hold  48  is fixed on a front frame of the transfer belt unit  3 . Further, a hold (not shown) is also provided on the fixing unit  6 . 
       FIG. 4B  is a view showing the printer body  1  with the front door  42  opened in the direction indicated by an arrow “b”. The fixing unit  6  is pulled out from the printer body  1  along guide/support rails (not shown) in the direction indicated by an arrow “c”. 
       FIG. 4C  is a view of the fixing unit  6  taken out from the printer body  1 . The housing of the fixing unit  6  is covered with a protection cover  40 . The protection cover is a member, which can be opened and closed about a hinge (not shown) in the direction indicated by an arrow “d”, as shown in  FIG. 4D . When the protection cover  40  is opened, components in the fixing unit  6  are exposed. 
       FIG. 4D  is a view showing the internal components of the fixing unit  6 . The internal components consist of a heat fixing roller  51  and a resiliently pressing roller (pressure roller)  52 . The heat fixing roller  51  is driven by a heat transfer belt  49 . The pressure roller  52  is resiliently pressed against the heat fixing roller  51  through the heat transfer belt  49 . 
     As shown in  FIG. 4D , the jammed paper  46  can be seen, which is eaten by the fixing unit  6  consisting of the rollers  51 ,  52  and the heat transfer belt  49 . As will be understood, the user will be able to easily remove the jammed paper  46  from the fixing unit  6 , when the internal components of the fixing unit  6  are exposed as shown in  FIG. 4D . 
     In the meantime, if the user tries to take out the fixing unit  6  from the printer body  1  immediately after the jammed paper has arisen, it will be in danger of getting burned, because the internal component (heat fixing roller  51 ) is heated to high temperatures as high as 110 to 180 degrees Centigrade in the fixing unit  6 . 
     The printer  1  according to the present embodiment 1 of the invention is provided with a special device, which allows the user to takeout the fixing unit  6  from the printer body  1  only after the heat fixing roller  51  has cooled down to temperatures, at which the user will not get burned. Hereinafter, the special device will be described. 
       FIG. 5A  is a schematic cross-sectional view showing the fixing unit  6  of the printer  1  in the printing operation.  FIG. 5B  is an enlarged schematic view showing the main portion of the fixing unit  6 .  FIG. 5C  is a detailed cross-sectional view of the fixing unit  6  taken on line A 1 -A 1  of  FIG. 5B . 
     As shown in  FIG. 5A  and  FIG. 5B , the fixing unit  6  is provided with the heat fixing roller  51  and the pressure roller  52  in a heat-resisting housing  53 . The heat transfer belt  49  is extended between the heat fixing roller  51  and a heat generating roller  55 . The heat generating roller  55  has a built-in heat-generating source  54  such as a halogen lamp. The housing  53  is provided with a cutout or window  53   a  at a position facing the heat-generating source  54 , and the cutout  53   a  is used for a temperature sensor  50  to measure a temperature of the heat-generating source  54 . The temperature sensor  50  is fixed to a position of the frame of the printer body  1 , the position of which faces the cutout  53   a  of the housing  53 . 
     As shown in  FIG. 5A , the paper  46  (shown in  FIG. 3B  and  FIG. 4D ) is guided to an inlet  56  of the fixing unit  6  from below upwards as indicated by an arrow “e”, and further guided between two guide plates  57  ( 57   a ,  57   b ), which are provided at both sides of the inlet  56 , finally to the fixing portion to be sandwiched between the heat fixing roller  51  and the pressure roller  52 . 
     The heat transfer belt  49  receives heat from the heat generating roller  55  heated by the heat-generating source  54 , and applies a radiational heating to the paper in the course along from the inlet  56  to the fixing portion to heat the same previously. In the fixing portion, the heat fixing roller  51  and the heat transfer belt  49  work together to apply heat directly onto the paper to fix the toner image on the paper with aid of the pressure roller  52 . 
     The paper with the toner image fixed thereon passes through the fixing portion and then is prevented from wrapping around the heat transfer belt  49  or the pressure roller  52  by separators  58  ( 58   a ,  58   b ) and finally guided to an outlet  59 . Further, as shown in  FIG. 5A , the paper is discharged along the conveyance path  61  indicated by a broken line by the discharging paired-rollers  36  and guided to the paper ejecting tray  37  shown in  FIG. 1 . 
     The heat transfer belt  49  is driven by the heat fixing roller  51 , and the heat generating roller  55  rotates in accordance with a turning movement of the heat transfer belt  49 . Meanwhile the pressure roller  52  rotates in accordance with a turning movement of the heat fixing roller  51  and the heat transfer belt  49 . 
     As shown in  FIG. 5C , the heat fixing roller  51  is supported on an internal frame  63  of the fixing unit  6  by means of a sleeve bearing  64 . More specifically, one end of a cylindrical shaft  62  of the heat fixing roller  51  is received by the sleeve bearing  64  fixed on the internal frame  63  of the fixing unit  6  and the axial movement of the shaft  62  is prevented by an end washer  65 . The other end (not shown) of the shaft  62  of the heat fixing roller  51  is engaged with a driving system (not shown) of the printer  1 . 
     As shown in  FIG. 5C , the pressure roller  52  is also supported on the internal frame  63  of the fixing unit  6 . More particularly, both ends (one end is not seen in  FIG. 5C ) of a cylindrical shaft  66  of the pressure roller  52 , both ends having the same structure, are received respectively by two bearings  67 ,  67  fixed on the internal frame  63  of the fixing unit  6 . A toric cam-follower  68  is pressed-fit on the outer race of the bearing  67  and the axial movement of the cam-follower  68  is prevented by an end washer  69 . 
     A bearing holding member  71  of the internal frame  63  for holding the bearing  67  is formed with a play for allowing the pressure roller  52  to be pressed against or released from the heat fixing roller  51 . The pressure roller  52  is always urged by bias means (not shown) to leave from the heat fixing roller  51 . A cam  73 , which rotates with a cam shaft  72  is in contact with the cam follower  68 . 
     When the printer  1  is in the printing operation, the cam  73  rotates to a position where the cam  73  presses the cam follower  68 , as shown in  FIG. 5B  and  FIG. 5C , whereby the pressure roller  52  is resiliently pressed against the heat fixing roller  51  with the heat transfer belt  49  held between them, as shown in  FIG. 5A ,  FIG. 5B , and  FIG. 5C . 
     The outer surface of the pressure roller  52  is made of a hard material member such as metals. Meanwhile, since the heat fixing roller  51  is made of foamed rubber adhered to a metal shaft, when pressed against the pressure roller  52 , the heat fixing roller  51  is slightly deformed. 
     In other words, as shown in  FIG. 5B , the pressure roller  52  slightly digs into the heat fixing roller  51  by a depth “f”, whereby the paper is tightly held between the pressure roller  52  and the heat fixing roller  51  and the toner image or printing image is firmly fixed on the paper. 
     The lower end portion of the camshaft  72  (as viewed in  FIG. 5C ) is eccentrically fixed to a stopper member  74 . The stopper member  74  rotates eccentrically in the same manner as the cam  73 . 
     When the printer  1  is in the printing operation and the cam shaft  72  eccentrically rotates to a position where the cam  73  presses the pressure roller  52  in the direction indicated by an arrow “g” towards the heat fixing roller  51  through the cam follower  68 , as shown in  FIG. 5B , the stopper member  74  eccentrically rotates in accordance with the rotation of the cam shaft  72 . 
     The portion (most distant portion) of the stopper member  74  which is most distant from the cam shaft  72  penetrates an opening  75  provided in a side wall of the housing  53  and comes into an opening  77  of a guide frame  76  of the printer body  1 , as shown in  FIG. 5C . Then, the fixing unit  6  cannot be removed from the printer body  1  in the front-back direction, that is, in the horizontal direction as indicated by an arrow “h” in  FIG. 5C . 
     As described above, when the cam  73  stays at the position where the cam follower  68  presses the pressure roller  52  against the heat fixing roller  51 , the stopper member  74  comes into the opening  77  of the guide frame  76  of the printer body  1 , preventing the fixing unit  6  from being removed from the printer body  1 , as clearly shown in  FIG. 5C . 
     In the present embodiment of the invention, when the temperature of the heat fixing roller  51  of the fixing unit  6  decreases, for example, to 100 degrees Centigrade or less, then the controlling unit (not shown) determines that the printer  1  is not in the printing operation, and makes the cam shaft  72  rotate by 180 degrees from the position shown in  FIG. 5C . It is apparent that a threshold value, based on which the controlling unit determines whether or not the printer  1  is in the printing operation is not limited to 100 degrees Centigrade, and the threshold value can be set to 100 degrees Centigrade or less, for example, the threshold value can be set to 70 degrees Centigrade. Therefore, it can be described in the present embodiment, that the temperature of the fixing unit  6  is 100 degrees Centigrade or less, based on which temperature the controlling unit determines that the printer  1  is not in the printing operation and makes the cam shaft  72  rotate 180 degrees from the position shown in  FIG. 5C . 
       FIG. 6A  is a view showing a relative position between the heat fixing roller  51  and the pressure roller  52  after the cam shaft  72  of  FIG. 5A  rotates 180 degrees from the position shown in  FIG. 5C .  FIG. 6B  is a view showing the cam shaft  72  of  FIG. 5B , which rotates 180 degrees from the position shown in  FIG. 5B .  FIG. 6C  is a view showing the cam shaft  72  of  FIG. 5C , which rotates 180 degrees from the position shown in  FIG. 5C .  FIG. 6C  is a detailed cross-sectional view of the fixing unit  6  taken on line A 2 -A 2  of  FIG. 6B . In  FIG. 6A ,  FIG. 6B , and  FIG. 6C , like component parts as those in  FIG. 5A ,  FIG. 5B , and  FIG. 5C  are designated by like reference numerals. 
     As described above, when the temperature of the heat fixing roller  51  of the fixing unit  6  decreases to 100 degrees Centigrade or less and it is determined that the printer  1  is not in the printing operation and the cam shaft  72  rotates 180 degrees from the position shown in  FIG. 5C , then the portion (closest portion) of the cam  73  closest to the cam shaft  72  will be in contact with the cam follower  68 , as shown in  FIG. 6B  and  FIG. 6C . 
     The pressure roller  52  is pressed by the bias means (not shown) in the direction (indicated by an arrow “i” in  FIG. 6B ) opposite to the heat fixing roller  51  by a difference between the most distant portion of the cam  73  and the closest portion of the cam  73 , wherein the most distant portion of the cam  73  is the portion of the cam  73  which is the most distant from the cam shaft  72 . Then, a distance “j” is left between the pressure roller  52  and the heat fixing roller  51  as shown in  FIG. 6B , and this distance “j” prevents the heat fixing roller  51  from deforming by an amount “f” indicated in  FIG. 5B . 
     Meanwhile, the most distant portion of the stopper member  74 , which penetrates the opening  75  provided in the housing  53  and stays in the opening  77  of the guide frame  76  of the printer body  1 , gets out of the openings  77  and  75 , and stays within the fixing unit  6 . 
     While the most distant portion of the stopper member  74  stays within the fixing unit  6 , the user is allowed to take out the fixing unit  6  from the printer body  1  to perform the jammed-paper removing operation ( FIG. 4A  to  FIG. 4D ). 
     The pressure roller  52  is brought from a pressing state to a releasing state at a time when the temperature sensor  50  has detected that the temperature of the heat fixing roller  51  decreases to a predetermined temperature or less (in the present embodiment, 100 degrees Centigrade or less), wherein, under the pressing state the pressure roller  52  is pressed against the heat fixing roller  51  and under the releasing state the pressure roller  52  is released from the heat fixing roller  51 . 
     An experiment taught that if the temperature of the heat fixing roller  51  decreases to 100 degrees Centigrade or less, the user can safely remove the fixing unit  6  from the printer body  1  in no danger of getting burned. 
     A mechanism will be described, which automatically releases the resilient pressure to be applied onto the Nip portion of fixing paired-rollers, when the fixing unit  6  is unlocked from its operating position. In general, the fixing unit  6  is provided with a cam mechanism for automatically pressing and/or releasing the pressure roller  52  against or from the heat fixing roller  51 . 
     A mechanism, which is composed of the stopper member fixed to the cam shaft, the opening provided in the housing of the fixing unit, and the opening provided in the guide frame of the printer body, serves as a mechanical locking mechanism for performing a locking and/or lock-releasing operation depending on the temperature of the fixing unit, wherein the locking operation does not allow the user to remove the fixing unit from the printer body and on the contrary the lock-releasing operation allows the user to remove the fixing unit from the printer body. 
       FIG. 7A  is a front view showing the fixing unit  6  and a driving system for driving its mechanical locking mechanism.  FIG. 7B  is a side view showing the fixing unit  6  and the driving system for driving its mechanical locking mechanism. The driving system  80  for driving the fixing unit  6  is mounted on the frame  81  of the printer body  1 , and has an electric motor  82 , which rotates in both normal and inverse directions. 
     The rotary shaft  83  of the motor  82  has a pinion  84  on its end. The pinion  84  meshes with a large-diameter gear  85  of a double-diameter reduction gear. A small-diameter gear  86  of the double-diameter reduction gear meshes with a first idle gear  87  and a second idle gear  88 . 
     The first idle gear  87  is provided with a one way clutch, and transmits only the normal rotational motion of the motor  82  to a first coupling gear  91 . Meanwhile, the second idle gear  88  is provided with a one way clutch  92 , and transmits only the inverse rotational motion of the motor  82  to a second coupling gear  93 . 
     A rotary shaft  94  of the first coupling gear  91  and a rotary shaft  95  of the second coupling gear  93  are supported on the frame  96  of the printer body  1 , and are provided with couplings  97 ,  98  on their ends, respectively. 
       FIG. 8A  is a view showing the fixing unit  6 , which is installed into the printed body  1  along the guide frame  76  of the printer body  1  shown in  FIG. 5C  and  FIG. 6C , and is in a state to be engaged with the driving system  80  for the fixing unit  6 .  FIG. 8B  is a view showing the fixing unit  6 , which is completely engaged with the driving system  80  for the fixing unit  6  (fixing-unit driving system). 
     The guide frame  76  of the printer body  1  and the housing  53  of the fixing unit  6  are not shown in  FIG. 8A  and  FIG. 8B . The part of the fixing unit  6 , which is not shown in  FIG. 5C  and  FIG. 6C , is shown in  FIG. 8A  and  FIG. 8B . The heat transfer belt  49  is omitted from the views of  FIG. 8A  and  FIG. 8B . 
     Concerning the heat fixing roller  51 , only its shaft  62  is indicated with its roller and bearing omitted in  FIG. 8A  and  FIG. 8B . In  FIG. 8A  and  FIG. 8B , like component parts as those in  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 6A ,  FIG. 6B ,  FIG. 6C , and  FIG. 7A ,  FIG. 7B  are designated by like reference numerals. 
     In  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 6A ,  FIG. 6B ,  FIG. 6C , the cam  73  is in contact with the cam follower  68 , but in  FIG. 8A  and  FIG. 8B  (the same as in  FIG. 9A  and  FIG. 9B ), the sleeve bearing  64  is used in place of the cam follower  68  for the sake of ease in drawing. 
     When the fixing unit  6  is installed deep into the printer body  1  (installing unit) in the direction indicated by an arrow “j” shown in  FIG. 8A  to engage with the fixing-unit driving system  80  as shown in  FIG. 8B , the coupling  97  of the first coupling gear  91  of the fixing-unit driving system  80  engages with a coupling  101  of the cylindrical shaft  62  of the heat fixing roller  51  of the fixing unit  6 . 
     A driving gear  101  is fixed between the portion of the internal frame  63  supporting the cylindrical shaft  62  and the coupling  101 . A driven gear  103  is fixed on the cylindrical shaft  66  of the pressure roller  52 . 
     A coupling  98  of the second coupling gear  93  of the fixing-unit driving system  80  engages with a coupling  104  of the cam shaft  72  of the fixing unit  6 . The both ends of the cam shaft  72  (Refer to  FIG. 9A  and  FIG. 9B ) are supported on the internal frame  63  by means of bearings  105 ,  105 . 
       FIG. 9A  is a view showing the fixing unit  6 , which is installed to the installing unit of the printer body  1  (the same as in  FIG. 6C ).  FIG. 9B  is a view showing the fixing unit  6 , which is installed and locked to the installing portion of the printer body  1  (the same as in  FIG. 5C ). In  FIG. 9A  and  FIG. 9B , like component parts as those in  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 6A ,  FIG. 6B ,  FIG. 6C , and  FIG. 8A ,  FIG. 8B  are designated by like reference numerals. 
       FIG. 10  is a block diagram of the electric unit  30  of the printer  1  including the controlling unit. As shown in  FIG. 10 , the electric unit  30  includes CPU (Central Processing Unit)  106 . 
     CPU  106  is connected with an interface controller (I/F-CONT)  107  and a printer controller (PR-CONT) via a data bus. The PR-CONT is further connected with a printer printing-unit  109 . 
     Further, CPU  106  is connected with ROM (Read Only Memory)  110 , EEPROM (Electrically Erasable and Programmable ROM)  111 , an operation panel  112  of an operation unit, and a sensor unit  113 , to which outputs are sent from sensors disposed in various units. 
     A system program is stored in ROM  110 , and CPU  106  controls operations of the various units in accordance with the system program stored in ROM  110 . Each unit performs its process under control of CPU  106 . 
     I/F-CONT  107  receives printing data sent from host apparatuses, for example, such as personal computers, and converts the received printing data into bit map data and further expands the bit map data in a frame memory  114 . 
     Storing areas for the respective colors of Black (K), Magenta (M), Cyan (C), and Yellow (Y) are prepared in the frame memory  114 . These pieces of image data of the respective colors are expanded on the corresponding storing areas of the frame memory  114 . 
     The image data expanded on the frame memory  114  is supplied to PR-CONT  108 , and further supplied to the printer printing-unit  109 . 
     The printer printing-unit  109  functions as an engine unit, and performs a drive controlling operation of a belt driving unit  115 . The belt driving unit  115  drives the driving roller  21  of the transfer belt unit  3  under control of PR-CONT  108  to drive the transfer belt  8 . 
     Further, the printer printing-unit  109  performs the drive controlling operation of a conveying-mechanism driving unit  116 . The conveying mechanism driving unit  116  drives a conveying mechanism consisting of driven units including the paper take-up rollers  31 , paper-sending rollers  32 , paper-handling rollers  33 , stand-by conveying paired-rollers  34 , paper ejecting paired-rollers  38 , photoreceptor drum  10 , and the heat fixing roller  51  of the fixing unit  6 . 
     The printer printing-unit  109  controls an output voltage of a power supplying unit  117 . The power supplying unit  117  applies electric power to an image forming system including the charge rollers  12 , optical writing heads  13 , primary transfer rollers  20 , secondary transfer rollers  35 , and also to the built-in heat-generating source  54  of the heat generating roller  55  in the fixing unit  6 . 
     The printer printing-unit  109  controls a motor driving unit  118 . The motor driving unit  118  watches an output sent from the temperature sensor  50  through the senor unit  113  and CPU  106  to judge whether or not the temperature of the fixing unit  6  reaches a predetermined threshold value (for example, 100 degrees Centigrade), and makes the electric motor  82  rotate in the normal or inverse direction depending on whether or not the temperature of the fixing unit  6  has reached the threshold value. 
     Plural pieces of image data of four colors, Black (K), Magenta (M), Cyan (C), and Yellow (Y), output from PR-CONT  108  are supplied to the printer printing-unit  109 , and further supplied to the corresponding optical writing heads  13  (shown in  FIG. 1 ), respectively. 
     Each of the optical writing heads  13  forms an optical latent image on the outer periphery of the photoreceptor drum  10  based on the image data sent from the printer printing-unit  109 , wherein the photoreceptor drum  10  is previously initialized by the charge rollers  12 . The developing device  9  develops the electrostatic latent image on the photoreceptor drum  10  to a toner image. 
     The toner images developed on the photoreceptor drum  10  are successively transferred onto transfer belt  8  by the primary transfer rollers  20  to be superimposed on the former image in order of the toner images of the colors, Yellow (Y), Magenta (M), Cyan (C), and Black (K) as seen from the upper stream. 
     The transfer belt  8  conveys the superimposed toner images to the secondary transferring unit  23 . The secondary transferring unit  23  transfers the superimposed toner images onto the paper (secondary transfer). The paper with the toner image transferred is sent to the fixing unit  6 . The fixing unit  6  applies heat and pressure to the toner image to fix the same on the paper. 
     Even if the user should try to take out the fixing unit  6  from the printer body  1  due to the jammed paper arisen in the course of the image forming process, the fixing unit  6  is kept in the locked state and cannot be taken out from the printer body  1  until the temperature of the fixing unit  6  has decreased to 100 degrees Centigrade or less. 
       FIG. 11  is a flow chart of the normal printing process performed by the printer  1  according to the present embodiment of the invention.  FIG. 12  is a flow chart of a process performed at the occurrence of the jammed paper. These flow charts are for explaining processes performed by the controlling unit to control the operation of the mechanical locking mechanism coordinating with the pressure roller  52  of the fixing unit  6 , depending on the temperature of the fixing unit. The processes will be described with reference to  FIG. 7A ,  FIG. 7B  to  FIG. 9A ,  FIG. 9B . 
     At step S 1  in the flow chart of  FIG. 11 , in general, the pressure roller  52  is released from the transfer belt  62  and the heat fixing roller  51  at the initial time, and stays at a position shown in  FIG. 8A  or  FIG. 9A . That is, the fixing unit  6  can be taken out from the installing unit of the printer body  1  (step S 1 ). 
     The controlling unit confirms based on a signal sent from a position detecting sensor (not shown) that the fixing unit  6  has been correctly installed on the installing unit (step S 2 ), and turns on the power to the fixing unit  6  (step S 3 ). In these processes, the motor driving unit  118  is brought into a controlling condition for driving the motor  82 . 
     Then, the motor rotates in the inverse direction to bring the pressure roller  52  to the position, where the pressure  52  is resiliently pressed to the heat fixing roller  51  (step S 4 ). In this process, the first idle gear  87  transmits only the normal rotational motion of the motor  82  to the first coupling gear  91 , as described with reference to  FIG. 7A  and  FIG. 7B . In other words, when the motor  82  rotates in the inverse direction, the first coupling gear  91  does not move. Therefore, no rotational movement is transmitted to the shaft  62  and the heat fixing roller  51  stays still in  FIG. 8B . 
     Meanwhile, since the second idle gear  88  transmits only the inverse rotational motion of the motor  82  to the second coupling gear  93 , the cam shaft  72  is made to rotate 180 degrees by the couplings  98  and  104  from the state shown in  FIG. 9A  to the state shown in  FIG. 9B . The periphery of the most distant portion of the cam  73  is in contact with the sleeve bearing  64 , whereby the pressure roller  52  is brought to the position by the sleeve bearing  64 , where the shaft  66 , that is, the pressure roller  52  is resiliently pressed against the heat fixing roller  51 . To rotate the cam shaft  72  by 180 degrees, a rotational ratio of each of the pinion  84  of the motor  82 , the large-diameter gear  85 , the small-diameter gear  86 , the second idle gear  88 , the second coupling gear  93 , and the couplings  98  and  104  is previously calculated, and the number of rotations of the motor  82  is determined, and then the motor  82  is made to rotate by the determined number of rotations. In this case, for instance, it is also possible to make the camshaft  72  rotate just by 180 degrees by making the cam shaft  72  rotate little by little, while a sensor is detecting a rotational position by reading a marker on either of the cam shaft  72 , the cam  72  and the stopper member  74 . 
     Further, the controlling unit makes the motor  82  rotate in the normal direction, and supplies the power to the heat-generating source  54  of the heat generating roller  55 , and watches a value shown by the temperature sensor  50  until the temperature of the heat-generating source  54  increases to an appropriate value (step S 5 ). 
     In this process, since the first idle gear  87  transmits only the normal rotational motion of the motor  82  to the first coupling gear  91  as described above, the shaft  62 , that is, the heat fixing roller  51  is driven by means of couplings  97  and  101  in  FIG. 8B . 
     Meanwhile, the second idle gear  88  transmits only the inverse rotational motion of the motor  82  to the second coupling gear  93 . In other words, when the motor  82  rotates in the normal direction, the second coupling gear  93  does not move. Therefore, no rotational movement is transmitted to the cam shaft  72  and the pressure roller  52  keeps the pressing state to the heat fixing roller  51  and is driven in accordance with the rotational movement of the heat fixing roller  51  through the driving gear  102  and the driven gear  103 . 
     Thereafter, when the motor  82  keeps the rotation in the normal direction and the temperature of the fixing unit  6  has reached a predetermined value, the printer  1  becomes ready to start a process of printing on the paper (step S 6 ). The printer  1  performs and finishes the printing process (step S 7 ). 
     During the printing process, the controlling unit keeps watching the output value of the temperature sensor  50 , and repeatedly performs turn-on and turn-off operations of the power to the heat-generating source  54 , thereby increasing and/or decreasing the temperature with an appropriate threshold value at its center, and controlling the temperature of the fixing unit  6  to keep the same within the allowable range of the appropriate temperature. 
     When the printing process has finished, the controlling unit sets the printer  1  to the idle state (step S 8 ), and further brings to the halt state (step S 9 ). In the halt state, the controlling unit keeps watching the output of the temperature sensor  50  until the temperature of the fixing unit  6  decreases to a predetermined temperature (for example, 100 degrees Centigrade) or less (step S 10 ). 
     When it is confirmed that the temperature of the fixing unit  6  has decreased to the predetermined temperature or less, the controlling unit makes the motor  82  rotate in the inverse direction to release the pressure roller  52  from the heat fixing roller  51  (step S 11 ). As described above, when the motor  82  rotates in the inverse direction, the heat fixing roller  51  does not rotate. As a result, the pressure roller  52  does not move, too. Only the cam shaft  72  rotates 180 degrees from the state shown in  FIG. 9B , and moves to the state shown in  FIG. 9A . 
     The periphery of the closest portion of the cam  73  faces the sleeve bearing  64 . And the sleeve bearing  64  moves to the lower end of the bearing holding member  71  of the internal frame  63 , which member having a play, and is released from the pressing state. 
     The fixing unit  6  with the pressure roller  52  released from the pressing state is shown in  FIG. 9A . The fixing-unit driving system  80  of the printer  1  shown in  FIG. 8A  and  FIG. 8B  is not shown in  FIG. 9A  and  FIG. 9B . 
     Now, a process, which is performed when a paper is jammed, will be described with reference to the flow chart shown in  FIG. 12 . The controlling unit detects the temperature of the heat fixing unit  6  based on the output signal from the temperature sensor  50  (step S 101 ). The controlling unit judges whether or not the temperature of the heat fixing unit  6  has decreased to the predetermined temperature (for example, 100 degrees Centigrade) or less (step S 102 ). 
     When it is determined that the temperature of the heat fixing unit  6  has not decreased to the predetermined temperature (NO at step S 102 ), the controlling unit keeps the standby state and makes a cooling fan (not shown) work to cool down the heat fixing unit  6  (step S 103 ). Again, the controlling unit returns to step S 102 . 
     When it is determined that the temperature of the heat fixing unit  6  has decreased to the predetermined temperature or less (YES at step S 102 ), the controlling unit makes the motor  82  rotate in the inverse direction to release the pressure roller  52  from the pressing state (step S 104 ). 
     As described above, when the motor  82  rotates in the inverse direction, the heat fixing roller  51  does not rotate. As a result, the pressure roller does not move, too. Only the cam shaft  72  rotates 180 degrees from the state shown in  FIG. 9B , and moves to the state shown in  FIG. 9A . 
     The periphery of the closest portion of the cam  73  faces the sleeve bearing  64 . And the sleeve bearing  64  moves to the lower end of the bearing holding member  71  of the internal frame  63 , which member having a play, and is released from the pressing state. 
     In this state of the printer  1 , the controlling unit displays an indication of “Fixing unit can be taken out” on an liquid crystal displaying device in the operation panel  112  on the top of the printer body  1 , informing the user that the user can safely take out the fixing unit  6  from the printer body  1 . 
     The mechanical locking mechanism and the controlling system according to the present embodiment can resolve inconveniences, in which the users get their hands burned by the hot heat fixing unit  6  while taking out the heat fixing unit  6  from the printer body  1  for maintenance purpose. 
     The cam shaft takes a time before it releases the locking of the stopper member, in other word, the heat roller needs some waiting time before its temperature decreases to the predetermined value or less (several seconds to ten and several seconds are needed depending on the state of the fixing unit). 
     But, needless to say, safety receives the highest priority. If this “safety first” will be understood by the users, the inventor of the present invention believes the problem of the waiting time will be accepted by the users as an allowable matter. 
     Having described and illustrated the principles of the present invention by reference to one preferred embodiment, it should be apparent that the preferred embodiment may be modified in arrangement and detail without departing from the principles disclosed herein and that it is intended that the invention be construed as including all such modifications and variations insofar as they come within the spirit and scope of the subject matter disclosed herein.