Abstract:
Disclosed a tandem type color image forming apparatus and a color image forming method to reduce the cost of production without lowering the transfer efficiency. An electric potential setting member ( 170 ) is disposed in the tandem color image forming apparatus. The electric potential setting member ( 170 ) sets the electric potential difference between the front and back faces of a dielectric belt ( 160 ) in tandem image forming units ( 110  to  140 ) to approximately 0V before a transfer material ( 100 ) is adsorbed. This enables the electric potential difference between the front and back faces of the dielectric belt ( 160 ) in the respective color transfer positions to be set to approximately 0V. Therefore, there is a wide range of selecting the dielectric belt types, and the lowering of the transfer efficiency can be prevented, which is caused by the lowering of the resistance value of the dielectric belt surface after running.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a color image forming apparatus for forming a color image on a sheet by using toners, and a color image forming method thereof, and more particularly to a color image forming apparatus which has a tandem type engine arranged a plurality of toner image forming units, and a color image forming method thereof.  
           [0003]    2. Description of the Related Art  
           [0004]    A tandem type color image forming apparatus as a color image forming apparatus, which has a plurality of toner image forming units parallel-disposed in a carrying path thereof, continuously forms the toner images with different colors on a sheet to enable a high-speed printing. FIG. 7 shows a conventional tandem type color electro-photographic apparatus.  
           [0005]    In FIG. 7, reference numbers  10 ,  20 ,  30  and  40  indicate the OPC (Organic Photoconductor) drums of the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units, respectively. The electrostatic latent images are formed on the OPC drums  10 , 20 , 30  and  40  and developed with Yellow-color, Magenda-color, Cyan-color, and Black-color toners by the unshown developing members in the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units.  
           [0006]    The developed toners are transferred onto a sheet  100  by the strength of an electric field, which has been generated between the OPC drums  10  to  40  and the sheet  100  by a voltage applied from transfer members  80  to  84  such as transfer rollers, etc. The sheet  100  is electrically charged by a sheet adsorption roller  60 , and then it is adsorbed onto a dielectric belt  50 .  
           [0007]    The sheet  100  is carried to the transfer positions of the OPC drums  10 ,  20 ,  30 , and  40  by the movement of the dielectric belt  50 , and all of the four colors are transferred onto the sheet  100 . Then, the sheet  100  is taken off the dielectric belt  50 , and the toner images on the sheet  100  are fixed by an unshown fixing member. Even when the four colors are transferred at different positions, the dielectric belt  50  adsorbs the charged sheet  100 , so that a high-quality color image can be formed without a position deviation of each color on the sheet  100 .  
           [0008]    As disclosed in U.S. Pat. No. 5,907,758 (Japanese Unexamined Published Patent 10-198120), and U.S. Pat. No. 6,021,286 (Japanese Unexamined Published Patent 11-161035), etc., in a conventional tandem type color electro-photographic process, the dielectric belt  50  is charged to a high electric potential such as approximately 1000 V by a charging device  70 . The reason why the dielectric belt  50  is charged to the high electric potential is explained. An electric potential difference between the sheet  100  and the OPC drums  10  to  40  can be increased for the charged electricity of the dielectric belt  50 , even when the transfer voltage applied to the four color toner transfer members  80 ,  81 ,  82 , and  83  is lowered. The strength of the electric field generated between the sheet  100  and the OPC drum is caused by increasing the potential difference between the sheet  100  and the OPC drum to a degree that no electric discharge occurs, so that the transfer efficiency can be improved.  
           [0009]    The above process is explained below, taking an example. The conductive brush  70  charges the dielectric belt  50  to 1000 V. At the same time, the sheet adsorption roller  60  charges the sheet  100  to adsorb the sheet  100  onto the dielectric belt  50 . At this time, the sheet  100  must be charged so that the potential difference between the front and back faces of the sheet  100  can be set to 2000 V.  
           [0010]    Then, the Yellow-color toners contained in the OPC drum  10  are transferred onto the sheet  100  in the Yellow-color toner transfer position. A voltage of −100 V (direct current) is applied to a transfer roller  80 . At this time, electric charges on the sheet move to a photosensitive body  10 . Therefore, the potential difference between the front and back faces of the dielectric belt  50  is lowered from 1000 V to 400 V.  
           [0011]    Then, the Magenda-color toners contained in the OPC drum  20  are transferred onto the sheet  100  in the Magenda-color toner transfer position. A voltage of 500 V (direct current) is applied to a transfer roller  82 . At this time, the electric charges move to a photosensitive body  20 . Therefore, the potential difference between the front and back faces of the dielectric belt  50  is lowered from 400 V to 200 V.  
           [0012]    Then, the Cyan-color toners contained in the OPC drum  30  are transferred onto the sheet  100  in the Cyan-color toner transfer position. A voltage of 700 V (direct current) is applied to a transfer roller  83 . At this time, the electric charges move to a photosensitive body  30 . Therefore, the potential difference between the front and back faces of the dielectric belt  50  is lowered from 200 V to 0 V.  
           [0013]    Finally, the Black-color toners contained in the OPC drum  40  are transferred onto the sheet  100  in the Black-color toner transfer position. A voltage of 900 V (direct current) is applied to a transfer roller  84 .  
           [0014]    In the above sequential transfer process, the potential difference between the surfaces of the sheet  100  and the photosensitive bodies  10  to  40  is always maintained at1200 V to obtain an even transfer efficiency.  
           [0015]    However, viewing from the characteristics of the dielectric belt, it is necessary to keep the charge carrying function to lower the transfer voltage until at least the four-color transfer process is completed, so the resistance value of the dielectric belt must be high and constant. Therefore, the dielectric belt needs to be selected in a limited and permissible range, so that there is a problem that it is difficult to lower the apparatus cost.  
           [0016]    It is known that when the running (printing) operation is executed to some degree, the surface-resistance on the dielectric belt as well as the electric charge carrying ability of the dielectric belt are lowered by the adsorption of impurities such as toners, etc. For example, FIG. 8 shows the result of measuring the electric potential fluctuations on the dielectric belt surface for the time (seconds) when a new dielectric belt (New Belt) before running and an old dielectric belt (Old Belt) after running during a specific time are charged to about 900 V.  
           [0017]    It is judged from this result that the electric charge carrying ability of the dielectric belt has been lowering. When the dielectric belt with the material characteristics in FIG. 8 is mounted onto the apparatus, the electric potential of the dielectric belt located in the toner transfer position is set to approximately 900 V before running, but is lowered to approximately 500 V after running, supposing that the electric potential of the dielectric belt is set to approximately 900 V and it takes two seconds for the dielectric belt to be carried from the charging roller  70  to the transfer position. When the electric potential of the dielectric belt is lowered, the effective electric potential difference between the sheet  100  and the OPC drums is also lowered, so there is a problem that the transfer efficiency is lowered, depending on the apparatus running time (operation time).  
           [0018]    Additionally, the tandem type color electro-photographic process contains many components, viewing from its characteristics that four image forming process units are parallel-disposed therein. For example, a general tandem type color electro-photographic process contains four sets of photosensitive bodies, photosensitive body chargers (containing the power source), exposure units, developing units, photosensitive body cleaning blades, transfer units (containing the power source), etc., respectively. Therefore, there is a problem that the tandem type color electro-photographic process is produced at higher costs than other color electrophotographic processes.  
           [0019]    To reduce the number of components, it is considered as an example that a transfer power source should be used commonly. However, as explained above, when the electric potential of the dielectric belt is set to a high electric potential such as 1000 V, etc., there are various electric potential differences between the front and back faces of the dielectric belt in the four-color transfer positions because of the material of the dielectric belt, the lowering of the surface-resistance value of the dielectric belt after running, the injection of electric charges from the photosensitive body into the dielectric belt. Therefore, the transfer efficiencies of the respective colors may be different from each other, when a voltage is applied from the same power source to the transfer unit.  
           [0020]    [0020]FIG. 9 conceptually shows the result of measuring the transfer efficiency when the transfer voltage of each color is changed, in the case that the belt  50  is charged to approximately 1200 V. As shown in FIG. 9, when the transfer voltages of all the colors are set to 1000 V, the Magenda-, Cyan-, and Black-color toner transfer efficiencies are 100%, but the Yellow-color toner transfer efficiency is 80%. Therefore, there is a problem that the transfer power source could not be used commonly and the apparatus could not be produced at lower costs.  
         SUMMARY OF THE INVENTION  
         [0021]    It is an object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which enable a tandem type engine to be produced at lower costs.  
           [0022]    It is another object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which prevent the transfer efficiency from being lowered by running.  
           [0023]    It is yet another object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which enable a transfer power source to be commonly used and the apparatus to be produced at lower costs.  
           [0024]    To attain the above objects of the present invention, a color image forming apparatus for forming a multi-color toner image to be a color image on a transfer material, or a color image forming method thereof comprises a plurality of image forming units for transferring toner images with different colors from an image carrier onto the transfer material, a dielectric belt for carrying the transfer material to sequentially pass the plurality of image forming units, a charging member for charging the transfer material to adsorb the transfer material onto the dielectric belt, and an electric potential setting member for setting the electric potential of the dielectric belt before the adsorption so as to prevent the electric potential difference between the front and back faces of the dielectric belt to affect each of the transfer operations.  
           [0025]    According to the present invention, the electric potential difference between the front and back faces of the dielectric belt in the tandem type image forming unit can be set to approximately 0 V before the dielectric belt adsorbs the transfer material so that the resistance value of the dielectric belt cannot affect the transfer operation. This enables the electric potential difference between the front and back faces of a dielectric belt to be set to approximately 0 V in the respective toner color transfer positions. Therefore, there is a wide range of selecting the dielectric belt types, and the lowering of the transfer efficiency can be prevented, which is caused by the lowering of the surface-resistance of the dielectric belt after running.  
           [0026]    According to the present invention, preferably by commonly using a transfer power source for transferring different color toners as well as by setting the above electric potential, the irregularities in the transfer efficiencies of the respective toner colors can be prevented, which are generated when the same transfer power source is commonly used, as well as the apparatus can be produced at lower costs.  
           [0027]    Furthermore, according to the present invention, the transfer efficiencies of all toner colors can be set to appropriate values, preferably by setting the voltage applied from the above transfer power source so that the electric potential difference between the latent image and the transfer medium surface can be set between 1100 V and 2600 V. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is the constitutional view of a color image forming apparatus as an embodiment of the present invention.  
         [0029]    [0029]FIG. 2 is the transverse sectional view of the transfer mechanism in FIG. 1.  
         [0030]    [0030]FIG. 3 is the explanatory view of the electric potential difference between the front and back faces of the dielectric belt in each transfer position of the present invention.  
         [0031]    [0031]FIG. 4 is the explanatory view of the transfer model in FIG. 2  
         [0032]    [0032]FIG. 5 is the equivalent circuit diagram of the dielectric belt in FIG. 2.  
         [0033]    [0033]FIG. 6 shows the relationship between the transfer voltage and the transfer efficiency in the present invention.  
         [0034]    [0034]FIG. 7 is the explanatory view of the prior art.  
         [0035]    [0035]FIG. 8 is the explanatory view of the electric potential fluctuations after the running in the prior art.  
         [0036]    [0036]FIG. 9 shows the relationship between the transfer voltage and the transfer efficiency in the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    [0037]FIG. 1 is a constitutional view of a color image forming apparatus as an embodiment of the present invention, and FIG. 2 is a transverse sectional view of the transfer mechanism in FIG. 1.  
         [0038]    As shown in FIG. 1, a tandem type color image forming apparatus  19  comprises four image forming units  110 ,  120 ,  130 , and  140 . The image forming unit  110 ,  120 ,  130 , or  140  is an electrophotographic unit comprising a photosensitive drum  410 ,  420 ,  430 , or  440 , a photosensitive body charger (containing a power source thereof)  300 , an exposure unit  310 , developing units  330  and  340 , a photosensitive body cleaning blade  350 , a transfer unit (containing a power source thereof)  510 ,  520 ,  530 , or  540 . Atonerbottle 320  for supplying toners with different colors to the developing unit is disposed in each of the four image forming units  110 ,  120 ,  130 , and  140 . For example, the Yellow-, Cyan-, Magenda-, and Black-color toners are contained in the toner bottles  320 , respectively.  
         [0039]    A sheet  100  as a transfer material is fed from a sheet feeding tray  200  or a manual insertion port  220 . A sheet adsorption roller  180  for charging the sheet  100 , an dielectric belt  160  such as PVDF, etc. for carrying the sheet  100 , and an electric potential setting roller  170  for setting the electric potential difference between the front and back faces of the dielectric belt  160  to approximately 0 V before adsorbing the sheet  100  are disposed in the tandem type color image forming apparatus  19 . A fixing unit  150  thermal-fixes a toner image on the sheet  100 , which has passed the image forming units  110 ,  120 ,  130  and  140 . A stacker  210  accommodates the sheet  100  after the toner image is thermal-fixed thereon.  
         [0040]    [0040]FIG. 2 explains a color image forming process. In FIG. 2, reference numbers  410 ,  420 ,  430 , and  440  indicate OPC (Organic Photoconductor) drums as photosensitive drums in the Yellow-, Magenta-, Cyan- and Black-color image forming process units  110 ,  120 ,  130 , and  140 , respectively, and reference numbers  510 ,  520 ,  530 , and  540  indicate transfer rollers thereof.  
         [0041]    An electrical latent image is formed on each of the OPC drums  410 ,  420 ,  430 , and  440 , as explained below. However, this latent image forming process is explained, taking an example of using minus-charged toners. The OPC drum is charged by the charger  300 , but in this embodiment, a brush charger is used. The OPC drums are charged up to −700 V by the conductive brush  300 .  
         [0042]    Then, an image formed portion on each of the OPC drums is exposed by light image, using an exposure unit  310  such as an LED array head, and the electric potential is lowered up to approximately −100 V. The developing unit (developing roller)  330  in FIG. 1 develops the electrostatic latent image formed on each OPC drum, using the minus-charged one-component Yellow-, Magenta-, Cyan- and Black-color toners. The developed toner image is transferred onto the sheet  100  to be carried, with a strength received from an electric field, which is generated between the OPC drums and the sheet with a voltage applied from the transfer units  510 ,  520 ,  530 , and  540  such as transfer rollers.  
         [0043]    This sheet  100  is charged by a sheet adsorption roller  180 , and adsorbed onto the dielectric belt  160 . The dielectric belt  160  is charged by the effects obtained from the sheet adsorption process, the four-color toner transfer process units, etc. However, in this embodiment, for example, the dielectric belt  160  is charged to 0 V by the electric potential setting unit  170  such as an electricity discharging brush  170 , etc. In this embodiment, the electricity discharging brush  170  is constituted of a conductive brush disposed around the SUS core metal. A voltage vpp of 2 kv is applied with a frequency of 800 Hz in the “sin” wave (alternating current) from a voltage source  172  to the electricity discharging brush  170 . A voltage of 500 V (direct current) is applied to the sheet absorption roller  180 .  
         [0044]    In this embodiment, the transfer members  510 ,  520 ,  530 , and 540  for transferring the Yellow-,Magenda-, Cyan- and Black-color toners are used as transfer rollers, and a transfer voltage is set to the same value in a range of 500 to 2000 V for each of the Yellow, Magenda, Cyan, and Black colors.  
         [0045]    The action of setting an electric potential of the front or back face of the dielectric belt  160  is explained, referring to FIGS. 4 and 5. FIG. 4 typically shows an example of the relationship in electric potential of the transfer process, when the transfer process is executed with minus-charged toners. In this example, “Vt” is the transfer voltage, “Vbelt” is the electric potential difference between the front and back faces of the dielectric belt, “Vpaper” is the potential difference between the front and back faces of the sheet, “V 1 ” is the electric potential of the sheet surface and “V 2 ” is the electric potential difference between the photosensitive body&#39;s surface and the sheet surface. The toner layer on the photosensitive body is transferred onto the sheet through an electric field generated by “V 2 ” as an electric potential difference between the photosensitive body&#39;s surface and the sheet surface.  
         [0046]    Supposing that Vpaper=200 V, Vbelt=1000 V, and Vt=−100 V are given, the electric potential of the sheet surface V 1  is represented as follows: 
         V 1 =Vt+Vbelt+Vpaper=1100 V  
         [0047]    Therefore, the electric potential difference “V 2 ” between the photosensitive body&#39;s surface and the sheet surface is represented below, considering the electric potential on the photosensitive body&#39;s surface as the reference electric potential: 
         V 2 =1100−(−100)=1200 V  
         [0048]    The toners on the photosensitive body are transferred onto the sheet  100  through an electric field generated by the above electric potential difference “V 2 .”  
         [0049]    [0049]FIG. 5 is an equivalent circuit diagram of the dielectric belt  160  in FIG. 4. After running, the resistance value of the surface of the dielectric belt  160  is lowered by the adhesion of impurities such as toners onto the dielectric belt  160 , etc. At the result, when the dielectric belt  160  is charged with electricity as conventional, as shown in FIG. 8, the electric potential of the surface of the dielectric belt  160  is lowered earlier, compared to that of the dielectric belt  160  before running.  
         [0050]    According to the present invention, as shown in FIG. 3, the electric potential difference “V 2 ” between the sheet surface and the photosensitive body&#39;s surface is not changed even when the resistance value of the surface of the dielectric belt  160  is lowered after running, because the electric potential difference between the front and back faces of the dielectric belt  160  is set to 0 V.  
         [0051]    In the prior art, the different transfer voltage “Vt” is set for each color. However, according to the present invention, the transfer voltages “Vbelt” in FIG. 4 are set almost same in the transfer positions of the respective colors by setting the electric potential difference between the front and back faces of the dielectric belt  160  to 0 V, so the transfer voltage “Vt” can be set to the same value for each color. Taking the model in FIG. 4, the transfer voltages “Vt” (VTY, VTM, VTC, VTB) of all the colors can be set to 900 V, because the electric potentials “V 1 ” of the sheet surfaces with all the colors are set to 1100 V, when the electric potentials of the front and back faces of the dielectric belt  160  are set to 0 V by the electricity discharging brush  170 .  
         [0052]    Then, the optimal range of transfer voltages “Vt” is explained, referring to FIG. 6. FIG. 6 shows the result of measuring the transfer efficiency when the transfer voltage is changed from −500 V to 2000 V, supposing that the electric potential of the dielectric belt is set to 0 V before the sheet is adsorbed thereto, as well as shows the relationship in transfer efficiency between the new belt and the old belt used for running. It is judged from the result of FIG. 6 that the transfer voltage ranges from 500 V to 2000 V when the generally admitted transfer efficiency is 80 % or more.  
         [0053]    When being converted to the transfer voltage into the electric potential difference between the OPC drums and the developing roller, the ranges of the electric potential difference become from 600 V to 2100 V, because the electric potential of the electrostatic latent image is −100 V. At this time, the sheet is charged so that the electric potential difference between the back and front faces thereof can be set to 500 V, and therefore the electrical potential difference ranges from 1100 V to 2600 V, depending on the electric potential difference between the sheet surface and the latent image.  
         [0054]    According to this embodiment, the lowering of the belt&#39;s electric potential, which is caused by the lowering of the electric charge carrying ability during running, can be prevented by presetting the electric potential difference between the front and back faces of the dielectric belt  160  to 0 V with the electric potential setting unit. This prevents the effective electric potential difference between the sheet and the OPC drums from being lowered by running, so the transfer efficiency can be rarely lowered. Additionally, the electric potentials of the transfer positions for the respective colors are set almost same on the sheet by setting the electric potential difference between the front and back faces of the dielectric belt  160  to 0 V, so that a transfer power source  190  can be used as a common power source, as shown in FIG. 2. Therefore, a color image forming apparatus can be produced at lower costs.  
         [0055]    The dielectric belt can be made of PVDF, polyimide, ETFE, polycarbonate, etc., and the image forming unit is not limited to the electrophotographic unit. Additionally, this embodiment has been explained above, taking the example in which the electric potential difference between the front and back faces of the dielectric belt  160  is set to 0 V by the electric potential setting unit. However, this electric potential difference need not be set accurately to 0 V, which is favorably set in the range that the resistance value of the dielectric belt does not affect the electric potential difference between the sheet surface and the photosensitive body&#39;s surface.  
         [0056]    The electric potential difference between the front and back faces of the electric belt before adsorbing the sheet is set to approximately 0 V, so that it can be prevented that the belt material affects the transfer efficiency and that the transfer efficiency is lowered by running.  
         [0057]    Additionally, when the transfer voltage is set to the same value for each color, the transfer efficiencies of the respective colors can be set almost same by setting the electric potential difference between the front and back faces of the dielectric belt to approximately 0 V.  
         [0058]    Additionally, a transfer efficiency over 80% can be obtained by setting the electric potential difference between the front and back faces of the dielectric belt to 0 V and by setting the electric potential difference between the transfer material surface and the electrostatic latent image in the range of 1100 to 2600 V.  
         [0059]    While the present invention has been particularly shown and described with reference to one preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention.