Abstract:
The fixing device of the present invention is provided in the conveying path to lead an image receiving medium having a developer image in the exit direction and includes a pressurizing/fixing roller pair for pressurizing and fixing the developer image by holding the image receiving medium, an endless belt shaped fixing belt having a first heater for heating and fusing the developer image in the conveying path before arriving at the fixing rollers, and an adjusting device for adjusting a gap between the image receiving medium and the fixing belt according to the characteristic of the image receiving medium passing the conveying path. Further, when the fixing belt overlaps a paper conveying unit for conveying a paper, a separating means is included for separating the fixing belt from the paper conveying unit when the fixing belt is not in the fixing operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device for fixing a developer image formed on an image receiving medium by heating and pressurizing a developer image by fixing rollers after heating and fusing the developer image by a fixing belt and a heating volume regulating method. 
     2. Description of the Related Art 
     For a fixing device that is used in an electro-photographic type copying machine, printer and the like for heating, pressurizing and fixing a toner image that is formed on a paper, a heat roller type fixing device for simultaneously heating and pressurizing a toner image by inserting a paper between a pair of fixing rollers having a heater or a belt type fixing device for heating, pressurizing and fixing a toner image by inserting a paper between a pair of fixing rollers after sufficiently heating and fusing a toner image formed on a paper by an endless belt shaped fixing belt is used. 
     On the other hand, a full color electro-photographic type image forming apparatus has come into wide use in recent years. This type of full color image forming apparatus uses paper in a wide range of thickness and also uses OHP paper, etc. and therefore, a high fixing property is demanded in order for achieving good color reproducibility for paper having various kinds of characteristics. 
     In order to obtain good full color reproducibility on such various kinds of paper, a fixing speed of a fixing device is so far controlled according to characteristics of paper or a heating temperature of a fixing device is controlled according to the characteristic of paper as disclosed in the Japanese Patent No. 10-274903. 
     However, in the case of a fixing device that controls a fixing speed, if a fixing speed was retarded by about 50%, the number of sheets to be copied will decrease by half and the performance drops as a result of the decrease in the number of copies. Furthermore, when intends to control a fixing speed only while maintaining other image forming processes unchanged, a distance from a peeling device to a fixing device requires at least a length of, for instance, A 3 size, that is the maximum paper size and the size of a fixing device will become large. 
     On the other hand, in the case of a heat roller type fixing device to control a fixing temperature, when paper in the range of, for instance, 80 g/m 2 ˜209 g/m 2  is usable, the paper thickness is classified into 4 kinds; ordinary paper of 80 g/m 2 ˜90 g/m 2 , thick paper  1  of 91 g/m 2 ˜105 g/m 2 , thick paper  2  of 106 g/m 2 ˜140 g/m 2  and thick paper  3  of 141 g/m 2 -209 g/m 2  and a fixing temperature of the fixing roller is controlled at 4 stages of 155° C., 165° C., 170° C. and 180° C. according to the classification of paper thickness. 
     As a definite example, when intended to change the image forming mode to the image forming mode for ordinary paper (80 g/m 2 ˜90 g/m 2 ) immediately after completing the copying in the image forming mode for thick paper  3  (141 g/m 2 ˜209 g/m 2 ), the fixing temperature of the fixing roller must be controlled to lower by 25° C. from 180° C. for the thick paper  3  to 155° C. for the ordinary paper. Normally, to lower a temperature of the fixing roller pair by 25° C., a time of about 3.5 min. is required even if the temperature is positively lowered by rotating the fixing roller pair. Therefore, operator must wait for a long time until the fixing roller reaches a proper temperature, the operability is lowered and furthermore, in order to control the fixing roller temperature positively, the fixing roller must be rotated even during the non-fixing time, and the life of the fixing roller is shortened. 
     Further, to improve the full color reproducibility of a belt type fixing device, it is necessary to sufficiently heat and fuse a toner by the fixing belt before pressurizing and fixing the toner by the fixing roller. So, in order to give a sufficient volume of heat to a toner, it is required to surely control the temperature of the fixing belt or secure a sufficient heating/fusing time using a long fixing belt. 
     Therefore, when intends to surely control the temperature of the fixing belt, a waiting time until the fixing belt reaches a proper fixing temperature when changing over the image forming mode will become long and the operability is deteriorated. 
     On the other hand, when the fixing belt is arranged to overlap the paper conveying unit to the fixing unit to downsize the main body of the fixing device when a long fixing belt is used, it was necessary to drive the paper conveying unit in order to prevent the deformation of the paper conveying unit by the heating from the fixing belt even when no paper is conveyed. Furthermore, in the case of a fixing device in which the paper conveying unit and the photosensitive drum are driven synchronously, the photosensitive drum must be driven jointly with the paper conveying unit as long as the fixing belt is heated even during the warming-up. Thus, the life of the paper conveying unit and the photosensitive drum was shortened and cost was increased. 
     Accordingly, in the case of a belt type fixing device, when an image forming mode is changed over so as to get good full color reproducibility regardless of the characteristic of a paper, it is desirable to be able to obtain the good operability without requiring a long waiting time and furthermore, to extend the life of the fixing device. Further, in the case of a belt type fixing device, it is desirable that the life of the paper conveying unit and the photosensitive drum is extended without impeding the downsizing of the main body of the device when a long fixing belt is used so as to get good full color reproducibility. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to obtain good full color image reproducibility without deteriorating the performance of the device due to decrease in the number of sheets to be copied when performing the image fixing suited to the characteristic of a paper. 
     Another object of the present invention is also to achieve good full color image reproducibility and obtain the high operability in a short waiting time irrespective of the image forming mode change-over to adapt to various kinds of paper. 
     A further object of the present invention is to extend the life of processing devices around the fixing device and reduce cost accordingly when intending the downsizing of the main body of the fixing device irrespective of use of a long fixing belt in order for sufficiently heating and fusing a toner image. 
     According to the present invention, there is provided a fixing device comprising pressuring/fixing means that is provided in a conveying path for leading an image receiving medium having a developer image in the exit direction for pressuring/fixing the developer image by holding the image receiving medium; endless belt shaped heating/fusing means having a first heating means for heating/fusing the developer image in the conveying path before arriving at the pressurizing/fixing means; and adjusting means for adjusting a gap between the image receiving medium and the heating/fusing means according to the characteristic of the image receiving medium passing the conveying path. 
     Further, according to the present invention, there is provided a fixing device comprising a pressurizing/fixing roller pair provided in the conveying path to lead an image receiving medium having a transferred developer image the exit direction for pressurizing/fixing the developer image by holding the image receiving medium; an endless belt shaped fixing belt having a first heater for heating/fusing the developer image in the conveying path before arriving at the pressurizing/fixing rollers; and an adjusting device for adjusting a gap between the image receiving medium and the fixing belt according to a characteristic of the image receiving medium passing the conveying path. 
     Further, according to the present invention, there is provided a heating volume adjusting method in a fixing device for heating/fixing a developer image formed on an image receiving medium by holding the image receiving medium by a pressurizing/fixing roller pair after heating and fusing the developer image by the fixing belt, comprising a step for adjusting a gap between the image receiving medium and the fixing belt according to a thickness of an image receiving medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematically showing an image forming unit of a full color printer to which a fixing device in a first embodiment of the present invention is installed; 
     FIG. 2 is an explanatory diagram schematically showing a fixing device to which the first embodiment of the present invention is applied; 
     FIG. 3 is an explanatory diagram for showing a cam of the fixing device shown in FIG. 2; 
     FIG. 4 is a diagram showing the fixing test results when a heat roller temperature and a second fixing roller temperature were controlled using ordinary paper in the fixing device shown in FIG. 2; 
     FIG. 5 is a diagram showing the fixing test results when a gap G 1  between a fixing belt and a fixing guide was adjusted and a heat roller temperature and a second fixing roller temperature were controlled using a thick paper  1  in the fixing device shown in FIG. 2; 
     FIG. 6 is a diagram showing the fixing test results when the gap G 1  between the fixing belt and the fixing guide was adjusted and the heat roller temperature and the second fixing roller temperature were controlled using a thick paper  2  in the fixing device shown in FIG. 2; 
     FIG. 7 is a diagram showing the fixing test result when the gap G 1  between the fixing belt and the fixing guide was adjusted and the heat roller temperature and the second fixing roller temperature were controlled using a thick paper  3  in the fixing device shown in FIG. 2; 
     FIG. 8 is a block diagram schematically showing an image forming unit of a full color printer to which the fixing device of the second embodiment of the present invention is installed; 
     FIG. 9 is an explanatory diagram schematically showing a fixing device at the time of fixing, to which the second embodiment of the present invention is applied; and 
     FIG. 10 is an explanatory diagram schematically showing a fixing device at the time of non-fixing, to which the second embodiment of the present invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in detail referring to the attached drawings. 
     FIG. 1 is a schematic diagram showing an image forming unit  10  of a full color printer, which is a full color electro-photographic device to which a belt type fixing device is installed, that is a first embodiment of the present invention. In the image forming unit  10 , 4 sets of recording devices  12 Y,  12 M,  12 C,  12 BK are deposited in a line along a transfer/conveyor belt  13 , that is a paper conveying unit, for forming color images in respective colors using yellow (Y), magenta (M), cyan (C) and black (BK) toners. 
     The recording devices  12 Y,  12 M,  12 C,  12 BK are in the same structures and therefore, the recording devices will be described referring to the yellow (Y) recording device  12 Y provided at the former stage and other recording devices  12 M,  12 C and  12 BK will be assigned with the same reference numerals and subscripts showing respective colors to the same portions and the explanations thereof will be omitted. 
     The recording device  12  has a photosensitive drum  14 Y and around it, there are arranged a charging device  16 Y, an exposing device  17 Y to apply yellow (Y) light signal, a developing device  18 Y, a transferring roller  20 Y, a cleaning device  21 Y, and a charge eliminating device  22 Y orderly along the rotating direction of the photosensitive drum  14 Y. The transferring roller  20 Y is arranged facing the photosensitive drum  14 Y via the transferring/conveyor belt  13 . 
     Further, below the recording devices  12 Y,  12 M,  12 C and  12 BK, there are a paper feed cassette device  24  for housing sheets of paper P that are image receiving media, a pickup roller  26  that picks up sheets of paper P from the paper feed cassette device  24  and feeds to the transferring/conveyor belt  13  at a proper timing, a feeding roller  27  and an aligning roller  28 . The conveying speed of a paper P by the aligning roller  28  and the transfer/conveyor belt  13  is set equal to the peripheral speed of the photosensitive drums  14 Y,  14 M,  14 C and  14 BK. 
     Further,  23  is a manual paper feed roller to supply a paper P manually. Normally, the paper feed cassette device  24  supplies ordinary paper of 80 g/m 2 ˜90 g/m 2 , and thick paper  1  of 91 g/m 2 ˜105 g/m 2 , thick paper  2  of 106 g/m 2 ˜140 g/m 2  and thick paper  3  of 141 g/m 2 ˜209 g/m 2  are supplied by the manual paper feed roller  23 . 
     Further, at the downstream side of the transferring/conveyor belt  13 , there are provided a belt type fixing device  30 , an exit roller pair  41  and a receiving tray  42 .  43  is a absorbing roller to give electric charge to the transferring/conveyor belt  13  to absorb a paper P,  44  is a transferring/conveyor belt charge eliminating roller, and  46  is a transferring/conveyor belt cleaner. 
     Next, the belt type fixing device  30  will be described in detail. The fixing device  30  has a first fixing roller  31  having a rotary shaft  31   a  that is extending parallel to the paper P conveying surface and a first heater  32  comprising a halogen lamp, and an endless type fixing belt  34  is put over the first fixing roller  31  and a heat roller  33  that is installed so that the axial distance to the first fixing roller  31  becomes 80 mm. 
     Further,  36  is a second fixing roller that has a second heater  37  comprising a halogen lamp and is pressurized and brought in contact with the first fixing roller  31  at a pressing force 60 kg by a pressing part (not shown) via the fixing belt  34  and comprises a pressing/fixing roller pair jointly with the first fixing roller  31  for heating, pressurizing and fixing a toner image by clamping a sheet of paper P. 
     The first fixing roller  31  has an aluminum core metal in diameter 38 mm covered by an 1 mm thick highly heat resisting silicon rubber, and the heat roller  33  is made of an aluminum in diameter 40 mm having the first heater  32  at the core. The fixing belt  34  is made of an electro-formed 40 μm thick nickel coated with 240 μm thick silicon rubber. The second fixing roller  36  has an aluminum core metal in diameter 40 mm having the second heater  37  covered by a 30 μm thick heat resisting silicon rubber. 
     Normally, the first fixing roller  31  is set at a hardness lower than the hardness of the second fixing roller  36  so that a sheet of paper P does not wind around the first fixing roller  31  and is ejected facing downward. The first fixing roller  31  is rotated at a predetermined speed in the arrow direction “v” shown in the figure by a motor, etc. (not shown) and turns the fixing belt  34  in the arrow direction “w”. Following the rotation of the fixing belt  34 , the second fixing roller  36  is rotated in the arrow direction “x”. The rotational speed of the fixing belt  34  is set at 100 mm/s. 
     The heat roller  33  and the second fixing roller  36  control the surface temperatures of the fixing belt  34  and the second fixing roller  36  to prescribed temperatures by turning the first and second heaters  32  and  37  ON/OFF, respectively. 
     Below the fixing belt  34 , there is provided a fixing guide  38  that is an adjusting device and a supporting guide able to oscillate in the arrow directions “s” and “t” shown in FIG. 2, centering around a supporting point  38   a  by a cam  40 . The cam  40  is rotated as controlled by a signal from a CPU of the main body of a full color printer, adjusts the position of the fixing guide  38  and adjusts a gap between the fixing belt  34  and the fixing guide  38  according to a thickness of a paper P and adjusts the quantity of heat for heating and fusing a paper P. 
     That is, as shown in FIG. 3, the cam  40  is in such the structure that it is able to position  4  points of a, b, c and d at the positions where they are brought in contact with the fixing guide  38 , and it is its home position where Point a is at the upper part and in contact with the fixing guide  38 . At this time, the fixing guide  38  is at a position equivalent to the lowest position and the gap G 1  between the fixing belt  34  and the fixing guide  38  becomes the maximum. The height of the fixing guide  38  when it is at this home position and the gap G 1  between the fixing belt  34  and the fixing guide  38  becomes the maximum is the height of the fixing guide  38  when fixing ordinary paper of 80 g/m 2 ˜90 g/m 2 . 
     Regarding other heights of the fixing guide  38 , the positions where Points b, c and d of the cam  40  are brought in contact with the fixing guide  38  are the heights of the fixing guide  38  when fixing the thick paper  1  of 91 g/m 2 ˜105 g/m 2 , the thick paper  3  of 141 g/m 2 ˜209 g/m 2  and the thick paper  2  of 106 g/m 2 ˜140 g/m 2 , and the gap G 1  between the fixing belt  34  and the fixing guide  38  is adjusted according to a thickness of paper in each class. Further, the fixing guide  38  is adjusted to respective heights according to a thickness of each paper and the heating volume to be applied to a paper P is adjusted and then, an image is fixed in respective paper thickness mode. 
     Next, the actual temperature control of the heat roller  33  and the second fixing roller  36  by the first and second heaters  32  and  37 , and the position adjustment of the fixing guide  38  will be described in detail. 
     Generally, in the case of ordinary paper of 80 g/m 2 ˜90 g/m 2 , if the fixing guide  38  is brought too close to the fixing belt  34 , there will be caused such a phenomenon that the trailing edge of a paper P is drawn to the fixing belt  34  by static electricity and a toner image that is not yet fixed is disordered and therefore, the gap G 1  between the fixing belt  34  and the fixing guide  38  is required to be at least 10 mm. 
     Accordingly, when fixing tests of ordinary paper were conducted and the fixing property was checked with the gap G 1  between the fixing belt  34  at the home position and the fixing guide  38  set at 10 mm, the results shown in FIG. 4 were obtained. That is, when the temperatures of the heat roller  33  were at 150° C. and 160° C. and the temperatures of the second fixing roller  36  were 140° C. and 150° C., good fixing property could be obtained without causing offset or jamming of paper P. Further, when considering the copying of both sides, since a good image is obtained without causing offset at the temperature of the second fixing roller  36  lower than the temperature of the heat roller  33  by 10° C., the temperature of the second roller  36  is set so that it becomes lower than the temperature of the heat roller  33  by 10° C. 
     Next, the position adjustment of the fixing guide  38  when fixing the thick paper  1  of 91 g/m 2 ˜105 g/m 2 , the thick paper  2  of 106 g/m 2 ˜140 g/m 2  and the thick paper  3  of 141 g/m 2 ˜209 g/m 2  will be described. 
     When the gap G 1  between the fixing belt  34  and the fixing guide  38  was set at 10˜6 mm and the best fixing temperatures of the heat roller  33  and the second fixing roller  36  for the thick paper  1  of 91 g/m 2 ˜105 g/m 2  were tested and the results were obtained as shown in FIG.  5 . 
     That is, when the gap G 1  between the fixing belt  34  and the fixing guide  38  is the same as that when fixing the ordinary paper, in order to obtain good fixing property, the temperatures of the heat roller  33  and the second fixing roller  36  must be set high. However, if the gap G 1  between the fixing belt  34  and the fixing guide  38  was set at 7 mm, it will be found that good fixing property is obtained without causing offset or jamming of paper even when the set temperatures of the heat roller  33  and the second fixing roller  36  are held at 160° C. and 150° C. that are the entirely same set temperatures at where good fixing property was obtained on ordinary paper as described above. 
     Then, when the gap G 1  between the fixing belt  34  and the fixing guide  38  was adjusted to 10˜3 mm and the best fixing temperatures of the heat roller  33  and the second fixing roller  36  at respective gaps were tested for the thick paper  2  of 106 g/m 2 ˜140 g/m 2  and the thick paper  3  of 141 g/m 2 ˜209 g/m 2  and the results were obtained as shown in FIG.  6  and FIG.  7 . 
     That is, it will be found that good fixing property is obtained without causing offset or jamming of paper if the gap G 1  between the fixing belt  34  and the fixing guide  38  is adjusted to 5 mm in the case of the thick paper  2  and to 3 mm in the case of the thick paper  3  even when the set temperatures of the heat roller  33  and the second fixing roller  36  are held at 160° C. and 150° C. that are the entirely same set temperatures at where good fixing property was obtained on ordinary paper as described above. 
     Further, in the above tests, to prevent the 1 mm thick silicon rubber coated on the core metal of the first fixing roller  31  from being peeled off or damaged, the temperature of the heat roller  33  was restricted to the maximum 200° C. Further, the gap G 1  between the fixing belt  34  and fixing guide  38  was restricted to the minimum 3 mm taking the conveyance of a paper P into consideration. 
     As the results of the above tests, it was revealed that when fixing ordinary paper of 80 g/m 2 ˜90 g/m 2 , the thick paper  1  of 91 g/m 2 ˜105 g/m 2 , the thick paper  2  of 106 g/m 2 ˜140 g/m 2  and the thick paper  3  of 141 g/m 2 ˜209 g/m 2 , the gap G 1  between the fixing belt  34  and the fixing guide  38  was adjusted to 10 mm, 7 mm. 5 mm and 3 mm, respectively, good fixing property will be obtained for paper in any thickness even when the temperature of the heat roller  33  is kept constantly at 160° C. and the second fixing roller  36  constantly at 150° C. 
     Accordingly, in this first embodiment, based on the above-mentioned test results, by rotating the cam  40  according to a thickness of a paper to be used, the positions of the fixing guide  38  and the fixing belt  34  are adjusted so that the gap G 1  between them is set at 10 mm by bringing Point a of the cam  40  in contact with the fixing guide  38  in the case of ordinary paper of 80 g/m 2 ˜90 g/m 2 , at 7 mm by bringing Point b of the cam  40  in contact with the fixing guide  38  in the case of the thick paper  1  of 91 g/m 2 ˜105 g/m 2 , at 5 mm by bringing the Point d of the cam  40  in contact with the fixing guide  38  in the case of the thick paper  2  of 106 g/m 2 ˜140 g/m 2  and at 3 mm by bringing the Point c of the cam  40  in contact with the fixing guide  38  in the case of the thick paper  3  of 141 g/m 2 ˜209 g/m 2 . 
     Further, the rotation of the cam  40  for adjusting the position of the fixing guide  38  is controlled by selecting first˜third change-over keys provided on the operation panel (not shown). That is, if any mode change-over key is not selected when image forming conditions are set when starting the image formation, it is recognized to be the image formation on ordinary paper and an image is fixed in the ordinary paper mode with the gap G 1  between the fixing guide  38  and the fixing belt  34  at 10 mm. When a first mode change-over key was selected, it is recognized to be the image formation on the thick paper  1  and an image is fixed in the thick paper  1  mode with the gap G 1  between the fixing guide  38  and the fixing belt  34  at 7 mm. Similarly, when the second or the third mode change-over key was selected, the image formation on the thick paper  2  or  3  is recognized and an image is fixed in the thick paper  2  mode with the gap G 1  between the fixing guide  38  and the fixing belt  34  at 5 mm or the thick paper  3  mode with the gap G 1  between the fixing guide  38  and the fixing belt  34  at 3 mm. 
     Further, in this first embodiment, the temperatures of the heat roller  33  and the second fixing roller  36  at the time of fixing are so set that they are constantly kept at 160° C. and 150° C., respectively. 
     Next, the operation will be described. When the start of the full color image formation is designated from the operation panel (not shown), operator sets image forming conditions such as the number of sheets of paper for image formation, magnification, etc. and when performing the copying by using paper other than ordinary paper of 80˜90 g/m 2 , selects paper P for forming an image from the thick paper  1  of 91 g/m 2 ˜105 g/m 2 , the thick paper  2  of 106 g/m 2 ˜140 g/m 2  and the thick paper  3  of 141 g/m 2 ˜209 g/m 2  by one of the first through third mode change-over keys on the operation panel (not shown) and selects the desired image forming mode. 
     When the copy key (not shown) is turned ON after setting required image forming conditions, image signals in respective colors are sent to the image forming units  10  and the yellow (Y), magenta (M), cyan (C) and black (BK) recording devices are operated at predetermined timings and toner images in respective colors, that are developer images, are formed on the photosensitive drums  14 Y˜ 14 BK. 
     That is, when taking the yellow (Y) recording device  12 Y as an example, with the rotation of the photosensitive drum  14 Y in the arrow direction “u”, the image forming processes are executed in order and the photosensitive drum is first electrified uniformly by the charging device  16 Y. Then, the exposing operation is executed on this uniformly electrified photosensitive drum  14 Y by the exposing device  17 Y and a latent image corresponding to the yellow (Y) image signal is formed on the photosensitive drum  14 Y. Thereafter, the photosensitive drum  14 Y is developed by the developing device  18 Y and a yellow (Y) toner image is formed on the photosensitive drum  14 Y. 
     Similarly, in the magenta (M), cyan (C) and black (BK) Recording devices  12 M.  12 C and  12 BK, toner images in respective colors are formed on the photosensitive drums  14 M,  14 C and  14 BK. 
     On the other hand, synchronous with the formation of toner images in respective colors on the photosensitive drums  14 Y,  14 M,  14 C and  14 BK, the pickup roller  26  or the manual paper feed roller  23  is driven and a paper P is supplied from the paper feed cassette device  24  or manually. This paper P is sent to the transferring/conveyor belt  13  after the leading edge is aligned by the aligning roller  28 . 
     The paper P sent to the transferring/conveyor belt  13  is applied with electric charge by the absorbing roller  43  and conveyed in the arrow direction “v” with the running of the transferring/conveyor belt  13  in the state electro-statically absorbed to the conveyor belt  13 . The paper P is first fed into a yellow toner image transferring position, that is, a position where the photosensitive drum  14 Y faces the transferring roller  20 Y with the transferring/conveyor belt  23  between them. The paper P is brought in contact with the yellow toner image formed on the photosensitive drum  14 Y at this yellow toner image transferring position. Then, the yellow toner image formed on the photosensitive drum  14 Y is transferred on the paper P from this state by the action of the transferring roller  20 Y. 
     The transferring roller  20 Y has semi-conductivity and supplies an electric field having the polarity reverse to the potential of the yellow toner image that is adhered electro-statically to the photosensitive drum  14 Y to the transferring/conveyor belt  13  through its back side. This electric field acts on the yellow toner image on the photosensitive drum  14 Y through the transferring/conveyor belt  13  and the yellow toner image is transferred on the paper P from the photosensitive drum  14 Y. The paper P with the yellow toner image thus transferred is then conveyed to the toner image transferring positions of the magenta recording device  12 M, the cyan recording device  12 C and the black recording device  12 BK in order. In the same manner as above, a magenta toner image, a cyan toner image and a black toner image are multi-transferred on the paper P in order in the similar manner as above and a full color toner image is formed. The paper P with the full color toner image formed is peeled off from the transferring/conveyor belt  13  and sent to a fixing device  30 , where a full color toner image formed in superposed colors is permanently fixed. 
     After this permanent fixing, the paper P is carried out on the receiving tray  32 . On the other hand, after the paper P is peeled off, the transferring/conveyor belt  13  is continuously driven to rotate and residual toner and paper powder are cleaned by a belt cleaner  46 . After this cleaning, the electric charge applied to the transferring/conveyor belt  13  is eliminated by the transferring/conveyor belt charge eliminating roller  44  so that the surface potential of the belt is kept constant. 
     Further, the photosensitive drums  14 Y,  14 M,  14 C and  14 BK are continuously rotated and driven after the toner images are transferred and residual toners and paper power are cleaned by the cleaners  21 Y,  21 M,  21 C and  21 BK. After this cleaning, the charge applied to the photosensitive drums  14 Y,  14 M,  14 C and  14 BK are eliminated by the charge eliminating device  22 Y,  22 M,  22 C and  22 BK so that the surface potential of the drums is kept constant and are put in the standby state for the next full color image forming process. 
     Next, actions of the fixing device  30  will be described. When the power source of the image forming unit  10  is turned ON and the warm-up is started, the first heater  32  and the second heater  37  are turned ON for preheating the fixing belt  34  and the second fixing roller  36 , the fixing belt  34  is rotated in the arrow direction “w” at a speed as high as, for instance, 100 mm/sec. and following this rotation of the fixing belt, the second fixing roller  36  is rotated in the arrow direction “x”. When the heat roller  33  reaches 160° C. and the second fixing roller  36  reaches 150° C. as a result of this warm-up, the image forming unit  10  becomes the ready state. 
     After the image forming unit  10  becomes the ready state, the fixing belt  34  is rotated at a speed as low as 50 mm/sec. and by turning the first heater  32  and the second heater  37  ON/OFF, the temperatures are controlled so as to maintain the heat roller  33  at 160° C. and the second fixing roller  36  at 150° C. Further, in this ready state, the fixing guide  38  is adjusted to the home position where Point a of the cam  40  is brought in contact with it and the gap G 1  with the fixing belt  34  is 10 mm. 
     Then, when the start of the image formation is designated and the full color image forming conditions are set through the operation panel, if the paper mode is the ordinary paper mode using ordinary paper of 80 g/m 2 ˜90 g/m 2  without selecting any of the first through third mode change-over keys (not shown), the rotation of the cam  40  is not controlled and the fixing guide  38  is kept at the home position. 
     On the other hand, when the first˜third change-over key (not shown) input is made when setting image forming conditions, the cam  40  is rotated corresponding to the mode change-over key input. When the first mode change-over key input is made, the cam  40  is rotated to bring Point b in contact with the fixing guide  38  and oscillates and adjusts the gap G 1  between the fixing guide  38  and the fixing belt  34  to 7 mm. 
     When the second mode change-over key input is made, the cam  40  is rotated to bring Point d in contact with the fixing guide  38  and oscillates and adjusts the gap G 1  between the fixing guide  38  and the fixing belt  34  to 5 mm. When the third mode change-over key input is made, the cam  40  is rotated to bring Point c in contact with the fixing guide  38  and oscillates and adjusts the gap G 1  between the fixing guide  38  and the fixing belt  34  to 3 mm. This rotational control of the cam  40  is completed in several seconds. 
     When the copying operation is started by turning the copy key (not shown) in this state, the fixing belt  34  and the second fixing roller  36  following thereto are rotated at the fixing speed of 100 mm/sec. in the fixing device  30 , sheets of paper P on which a full color toner image is transferred in the above-mentioned full color image forming process and conveyed on the transferring/conveyor belt  13  are supported and guided in the directions of the fixing rollers  31 ,  36  by the position adjusted fixing guide  38  according to the thickness of the paper P and the full color toner images on the sheets of paper P are heated and fused by the fixing belt  34  that is heated to 160° C. during this period. Further, the sheets of paper with the full color toner images heated and fused are inserted between the first fixing roller  31  and the second fixing roller  36  that is heated to 150° C. and the full color toner image is heated, pressurized and fixed. 
     That is, when fixing an image, as the heating volume to be applied to a paper P according to its thickness is adjustable by adjusting the gap G 1  between the fixing belt  34  and the fixing guide  38  according to a thickness of a paper P, a full color toner image is applied with a sufficient heating volume when passing through the gap G 1  and sufficiently heated and fused even when the image formation is executed using sheets of paper P in any thickness and then, heated, pressurized and fixed between the first fixing roller  31  and the second fixing roller  36 . Thus, the good fixing property is obtained on a paper P in any thickness. 
     Thus, by adjusting the gap G 1  between the fixing belt  34  and the fixing guide  38  by oscillating the fixing guide  38  according to a thickness of a paper, the heating volume to be applied to a paper P that is running on the fixing guide  38  can be adjusted easily in a very short time without adjusting the conveying speed of a paper P or adjusting the heating temperature by the fixing belt while maintaining the copying speed and fixing temperature constant. Accordingly, irrespective of difference in paper thickness, a full color image is obtained efficiently without deteriorating the performance of the image forming apparatus due to drop in the number of sheets of paper to be copied resulting from drop in copying speed. Furthermore, a long standby time is not required when changing over the image forming mode and the operability of the image forming apparatus can be improved as a result of the reduced waiting time. 
     Next, a second embodiment of the present invention will be explained. In the second embodiment, the heating volume to be applied to a paper P is regulated by adjusting a gap between the fixing belt and fixing guide by oscillating the fixing belt instead of the fixing guide that is used in the first embodiment for adjusting the gap according to thickness of a paper P. 
     Further, in this second embodiment, the fixing belt is extended more longer in order to obtain the good fixing property by sufficiently heating and fusing a full color toner image that is formed by superposing yellow (Y), magenta(M), cyan (C) and black (BK) toners in the first embodiment and on the other hand, when the fixing belt is partially overlapped on the end of the transferring/conveyor belt in order for preventing the image forming unit from becoming a large in size, the fixing belt is separated from the transferring/conveyor belt in order for preventing the transferring/conveyor belt from being heated and deteriorated by the fixing belt, when a toner image is not heated, fused and fixed by the fixing belt and at least no paper is conveyed and the transferring/conveyor belt is stopped to be driven. 
     Further, in this second embodiment, the same component elements as those in the first embodiment are assigned with the same reference numerals and the detailed explanations thereof are omitted. 
     FIG. 8 is a schematic diagram showing an image forming unit  50  of a full color printer that is a full color electro-photographic printer equipped with a belt type fixing device of the second embodiment of the present invention. In a fixing device  51 , the first fixing roller  31  and the heat roller  33  are installed so that a distance between their shafts becomes 120 mm and a fixing belt  52  is put over both rollers  31 ,  32 . The end of the fixing belt  52  of the heat roller  22  side is overlapped on the end f the transferring/conveyor belt  13 . Further,  57  is a stationary fixing guide. 
     The first fixing roller  31 , the heat roller  33  and the fixing belt  52  are supported in one united body by a frame  54  that is able to oscillate in the directions of arrow marks “m” and “n” centering around a supporting point  54   a  by a cam  53  and comprise a fixing belt unit  56 .  58  is a pressurizing spring to constantly press the frame  54  down against the cam  53 . 
     The cam  53  is controlled to rotate by a signal from a CPU of the main body of the full color printer so as to position five points of e, f, g, h and i at the locations where they contact the frame  54 , oscillates the frame  54  against the pressurizing spring  58 , adjusts the position of a gap G 2  between the fixing belt  52  and the stationary fixing  57  according to a thickness of a paper P and regulates the heating volume for sufficiently heating and fusing a paper P. Further, when the transferring/conveyor belt  13  is stopped, the cam  53  separates the heat roller  33  side end of the fixing belt  52  from the end of the transferring/conveyor belt  13  by oscillating the position of the frame  54 . 
     That is, as shown in FIG. 10, when Point e of the cam  53  is at the upper part and the frame  54  is fully oscillated in the arrow direction “m”, this position is the home position of the cam  53 . At this home position, the heat roller  33  side end of the fixing belt  52  is separated from the transferring/conveyor belt  13  by about 30 mm. On the other hand, as shown in FIG. 9, when Point i of the cam  53  is positioned at the upper part and the cam  53  is separated from the frame  54 , the gap G 2  between the fixing belt  52  and the fixing guide  57  is adjusted to minimum 3 mm so that the fixing belt  52  fixes the thick paper  3 . 
     Further, when Point f of the cam  53  is in contact with the frame  54 , the fixing belt  52  is adjusted to a position so that the gap G 2  with the fixing guide  57  becomes 10 mm so as to fix ordinary paper. Similarly, when Point g or h of the cam  53  is in contact with the frame  54 , the position of the fixing belt  52  is adjusted so that the gap G 2  with the fixing guide  57  becomes 7 mm or 5 mm so as to fix the thick paper  1  or  2 . 
     The rotation of the cam  53  for adjusting the position of the fixing belt  52  by oscillating the frame  54  is so controlled that Point e of the cam  53  is always positioned at the home position where it is in contact with the frame  54  when no image is formed and the transferring/conveyor belt  13  is stopped. On the other hand, during the copying operation, the rotation of the cam  53  is controlled by selecting fourth˜sixth mode change-over keys on the operation panel (not shown). That is, when the mode change-over key is not selected during the copying operation, it is recognized to be the image formation on ordinary paper and the cam  53  is rotated so that Point f is brought in contact with the frame  54 . When the fourth mode change-over key is selected, it is recognized to be the image formation on the thick paper  1  and the cam  53  is rotated so that Point g is brought in contact with the frame  54 . When the fifth mode change-over key is selected, it is recognized to be the image formation on the thick paper  2  and the cam  53  is rotated so that Point h is brought in contact with the frame  54 . When the sixth mode change-over key is selected, it is recognized to be the image formation on the thick paper  3  and the cam  53  is rotated so that Point i is positioned at the upper part and is separated from the frame  54 . 
     Next, the fixing process of the fixing device  51  will be described. When the power source of the image forming unit  10  is turned ON, the frame  54  is adjusted to position at the home position where Point e of the cam  53  is brought in contact with it, and the heat roller  33  side end of the fixing belt  52  is separated from the transferring/conveyor belt  13  by about 30 mm. In this state, the warm-up starts, the first and second heaters  32  and  37  are turned ON for preheating the fixing belt  52  and the second fixing roller  36 , the fixing belt  52  is rotated in the arrow direction “o” at a high speed of, for instance, 100 mm/sec. and following the rotation of the fixing belt  52 , the first fixing roller  36  is rotated in the arrow direction “p”. 
     When the heat roller  33  and the second fixing roller  36  reach 160° C. and 150° C., respectively as a result of this warm-up, the image forming unit  50  becomes the ready state. 
     After this ready state, the fixing belt  52  is rotated at a low speed of 50 mm/sec. and the heat roller  33  and the second fixing roller  37  are kept at 160° C. and 150° C., respectively by turning the first and second heaters  32  and  37  ON/OFF. 
     Then, when the start of the image formation is designated and the copy key (not shown) is turned ON, the recording devices  12 Y,  12 M,  12 C and  12 BK of the image forming unit  10  are operated at predetermined timings and the transferring/conveyor belt  13  is rotated in the arrow direction “v”. 
     On the other hand, in the fixing device  51 , if the mode change-over key (now shown) input was not made from the operation panel when setting the image forming conditions at the time to designate the start of image formation, the cam  53  is rotated to a position where Point f is brought in contact with the frame  54  from the home position, the gap G 2  between the fixing belt  52  and the fixing guide  57  is adjusted to 10 mm, and it becomes possible to fix an image on ordinary paper. Further, when the fourth or fifth mode change-over key (not shown) is set at the time when setting the image forming conditions, the cam  53  is rotated to a position where Point g or h of the cam  53  is brought in contact with the frame  54 , the gap G 2  between the fixing belt  52  and the fixing guide  57  is adjusted to 7 mm or 5 mm, and the fixing of the thick paper  1  or  2  becomes possible. 
     Further, when the sixth mode change-over key is set when setting the image forming conditions, the cam  53  is rotated to a position where Point i is positioned at the upper part and is separated from the frame  54 . As a result, the gap G 2  between the fixing belt  52  and the fixing guide  57  is adjusted to 3 mm and the fixing of the thick paper  3  becomes possible. Further, the rotation of the cam  53  is completed in several seconds. 
     Then, when the copy key (not shown) is turned ON and the copying operation starts, in the fixing device  51 , the fixing belt  52  and the following second fixing roller  36  are rotated at a fixing speed of 100 mm/sec., a full color toner image formed on the paper is sufficiently heated and fused by the fixing belt  34  that is heated to 160° C. while guiding and supporting a sheet of paper P conveyed by the transferring/conveyor belt  13  by the fixing guide  57  and a sheet of paper P having a sufficiently heated and fused toner image is passed between the first fixing roller  31  and the second fixing roller  36  that is heated to 150° C. and a full color toner image is heated, pressurized and fixed. 
     That is, because the heating volume applied to a paper P is adjustable according to a thickness of a paper P by adjusting the gap G 2  between the fixing belt  52  and the fixing guide  57  according to a thickness of a paper P when fixing, a full color toner image is applied with a sufficient heating volume and thoroughly heated and fused while passing through the gap G 2  even when an image is formed using any paper P, good fixing property is obtained on a paper in any thickness when heated, pressurized and fixed between the first and second fixing rollers  31  and  36 . 
     Thereafter, when the image forming process is completed, the recording devices  12 Y,  12 M,  12 C and  12 BK are stopped and the transferring/conveyor belt  13  is also stopped to rotate. At this time, in the fixing device  51 , the rotation of the cam  53  is so controlled that Point e is brought in contact with the frame  54  and the heat roller  33  side end of the fixing belt  52  is returned to the home position that is separated from the transferring/conveyor belt  13  by about 30 mm. Accordingly, although the transferring/conveyor belt  13  is stopped to run, the fixing belt  52  is separated from the transferring/conveyor belt  13  and the transferring/conveyor belt  13  will not be deformed partially by the heated fixing belt  52 . 
     Thus, by adjusting the gap G 2  between the fixing belt  52  and the fixing guide  57  by oscillating the fixing belt  52  according to a thickness of a paper P, the heating volume to be given to a paper P running on the fixing guide  57  can be adjusted easily in a short time while keeping the copying speed and the fixing temperature constant. Accordingly, irrespective of difference in a paper thickness, a full color image can be obtained at a high performance without deterioration of a performance of an image forming apparatus resulting from decrease in the number of copies due to reduced copying speed. Further, a long waiting time is not required when changing an image forming mode according to a paper thickness and it becomes possible to improve the operability by making a waiting time short. 
     Furthermore, when the fixing belt  52  is arranged partially to overlap the transferring/conveyor belt  13  so as not to make the image forming unit  51  large when using a long fixing belt  52  to extend a heating distance for sufficiently heating and fusing toners, the fixing belt  52  is always kept separated from the transferring/conveyor belt  13  as long as the transferring/conveyor belt  13  is stopped to run when no image is fixed. Therefore, it is possible to prevent the thermal deformation of the transferring/conveyor belt  13  by the heating of the fixing belt  52 . Accordingly, because the thermal deformation of the transferring/conveyor belt  13  is prevented, it is not necessary to excessively drive the transferring/conveyor belt  14  and the photosensitive drums  14 Y,  14 M,  14 C,  14 BK that are rotated synchronously with the transferring/conveyor belt  13  when no paper is conveyed. Thus, the life of the transferring/conveyor belt  13  and the photosensitive drums  14 Y,  14 M,  14 C,  14 BK can be extended and a low cost can be achieved as a result of the prevention of unnecessary driving. 
     Further, the present invention is not restricted to the above-mentioned embodiments but can be modified variously within the scope of the present invention. For instance, in the above embodiments, the heating volume to be given to a paper is regulated by adjusting the gap between the fixing belt and the supporting belt according to a thickness of paper; however, in order for regulating the heating volume to be given to paper, the gap is adjustable according to characteristics of paper such as ordinary paper and OHP paper. 
     Further, the present invention is also not restricted to thickness of paper to be used. When other paper than the paper used in the above embodiments is used and the gap cannot be made smaller than the specified widths in order to retain the paper conveying property, the gap may be set at the minimum width and on the other hand, the fixing belt and fixing roller may be heated up to proper temperatures so as to obtain good fixing property. Although a time is needed for the temperature regulation of the fixing belt and the fixing roller at this time, it is limited to adjust only the insufficient heating volume that cannot be covered by the gap and therefore, a time required for the temperature adjustment can be reduced remarkably more than before and a waiting time at the time of mode change-over also can be reduced remarkably and thus, the improved operability is obtained. 
     Further, in the second embodiment the deformation of the transferring/conveyor belt is prevented by oscillating the fixing belt in multiple stages and the gap between the fixing belt and the fixing guide is adjusted; however, the gap between the fixing belt and the fixing guide may be adjusted by oscillating the fixing guide and the fixing belt may be oscillated only when separating it from the transferring/conveyor belt for preventing the thermal deformation of the belt. Further, if the heating and fusing of a toner image by the fixing belt is sufficient enough, it may not be necessary to provide a heater to the fixing roller. 
     According to the present invention as described above in detail, the heating volume suited to various kinds of paper becomes adjustable easily in a very short time by adjusting the gap between the heating/fusing device and the image receiving medium, the operability when forming a full color image can be improved without requiring a long waiting time for changing an image forming mode and also, a full color image can be obtained in a high performance without causing drop in the number of image forming sheets. 
     Further, according to the present invention, as toners are sufficiently heated and fused, even when a part of a long heating/fusing device is arranged by overlapping a conveying device at the upper stream, the heating/fusing device is separated from the conveying device when no image is fixed and at least the conveying device is stopped, the thermal deformation of the conveying device can be prevented. Accordingly, when no paper is conveyed, it is not required to drive the conveyor belt and the photosensitive drums that are driven synchronously with the conveyor belt, and a long life of the conveying device and the photosensitive drums and low cost can be achieved.