Abstract:
An image forming apparatus, including an image carrier, an image forming section for forming toner image on the image carrier, an image transfer device for transferring the toner images from the image carrier to a transfer material, and a registration member for conveying the transfer material to a position where the image transfer device transfers the toner image in synchronization with the image formation conducted by the image forming section, wherein the registration member includes a drive roller having a core and a rubber layer formed on the core, and a driven roller coming into contact with the drive roller, wherein the drive roller satisfies formula (1), d/D≦57.6 L −1.2 , and formula (2), d÷0.5, where D is diameter of the drive roller, d is thickness of the rubber layer, and L is conveyance distance from the registration member to the image transferring position.

Description:
[0001]     This application is based on Japanese Patent Application No. 2005-127663 filed with the Japanese Patent Office, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to improvement of a registration member in an image forming apparatus which forms an image using electro-photographic process, that is, to improvement of a conveyance device which-conveys a transfer material, synchronizing the conveyance with image formation.  
         [0003]     In recent years, an image forming apparatus, which forms the image by the electro-photographic process, has been used in the field of printing such as POD (print on demand), accordingly a technology for printing the image on a transfer material at a high positioning accuracy has been strongly required in the market.  
         [0004]     In the image forming apparatus which forms the image by the electro-photographic process, toner images are formed on an image carrier, then the toner images on the image carrier are transferred onto the transfer material, and thereby final images are generated on the transfer material. During the above process, concerning the conveyance direction, the image position on the transfer material depends upon an operational accuracy of the registration member which conveys the transfer material in synchronization with the image formation on the image carrier, and in particular depends on the starting time and the conveyance rate of the transfer material, conducted by the registration member.  
         [0005]     However, due to changes of temperature in the image forming apparatus, the thermal expansion of the registration member causes changes of conveyance rate of the transfer material, which results in positional deviation in the image. Particularly in the case of two-sided image formation, there has been a problem in that the image position of the front surface differs from that of the reverse surface. In the image forming apparatus which forms the image by the electro-photographic process, one method employs the intermediate transfer device through which the images are transferred from an image forming section to the transfer material, while the other method directly transfers the images from the image forming section to the transfer material. The above mentioned positional deviation of the images has occurred in both methods.  
         [0006]     To this problem, Patent Document 1 proposes that the temperature of a drive roller, structured in a registration member, is detected, and the rotation rate of the drive roller is controlled based on the detected temperature.  
         [0007]     [Patent Document 1] Japanese Non-examined Patent Publication No. 2000-44083  
         [0008]     However, according to Patent Document 1, structure of the apparatus and its control are very complicated, and has less reliability as well as higher production cost.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention will now be described below.  
         [0010]     Item 1. An image forming apparatus, including, an image carrier, an image forming section for forming toner images on the image carrier, an image transfer device for transferring the toner image from the image carrier to a transfer material, and a registration member for conveying the transfer material to a position where the image transfer device transfers the toner image in synchronization with the image formation conducted by the image forming section, wherein the registration member includes a drive roller having a core and a rubber layer formed on the core, and a driven roller coming into contact with the drive roller, wherein the drive roller satisfies formulas (1) and (2), 
 
 d/D≦ 57.6  L   −1.2   Formula (1) 
 
d≧0.5  Formula (2) 
 
         [0011]     where, 
        D: diameter of the drive roller,     d: thickness of the rubber layer,     L: conveyance distance from the registration member to the position where the image transfer section transfers the toner image.        
 
         [0015]     Item 2. The image forming apparatus described in Item 1, wherein the drive roller satisfies formula (3), 
 
 d/D≦ 103.9 L −1.5   Formula (3) 
 
         [0016]     Item 3. The image forming apparatus described in Item 1, further including a sheet re-supplying section which turns over the transfer material carrying the image formed on one side, and then supplies the transfer material, showing the reverse surface, to the registration member.  
         [0017]     Item 4. The image forming apparatus described in Item 1, wherein the image carrier is a photosensitive device on which image is exposed by an exposure device.  
         [0018]     Item 5. The image forming apparatus described in Item 1, wherein the image carrier is an intermediate transfer device on which the toner image on the photosensitive device are transferred.  
         [0019]     Item 6. The image forming apparatus described in Item 1, wherein the driven roller is structured of a rigid body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a cross section of a color image forming apparatus, being an example of the embodiments of the image forming apparatus related to the present invention.  
         [0021]      FIG. 2  shows a conveyance path from an intermediate transfer rollers to a transfer position via a registration member.  
         [0022]      FIG. 3  is an operational timing chart of an exposure device and the registration member.  
         [0023]      FIG. 4  shows a transfer sheet on which the images are printed.  
         [0024]      FIG. 5  shows the structure of the registration member.  
         [0025]      FIG. 6  shows the transfer sheet on both surfaces of which the images were printed.  
         [0026]      FIG. 7  is a graph showing the experimental results.  
         [0027]      FIG. 8  shows another example of the image forming apparatus related to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The embodiments of this invention will be detailed below, however, the present invention is not limited to the present embodiments.  
         [0029]      FIG. 1  is a cross sectional view of a color image forming apparatus as an example of the embodiment related to the present invention.  
         [0030]     Color image forming apparatus  100 , referred to as a tandem type color image forming apparatus, includes image forming sections  10 Y,  10 M,  10 C and  10 K as the image forming devices, intermediate transfer device unit  7 , sheet supply and conveyance device  21 , and fixing device  24 .  
         [0031]     Original document reading device SC is mounted on an upper section of main section A of the image forming apparatus.  
         [0032]     Image forming section  10 Y, to form yellow toner images, includes charging device  2 Y, exposure device  3 Y, developing device  4 Y, first transfer roller  5 Y and cleaning device  6 Y, all of which are arranged around drum-shaped photo-conductor  1 Y.  
         [0033]     Image forming section  10 M, to form magenta toner images, includes charging device  2 M, exposure device  3 M, developing device  4 M, first transfer roller  5 M and cleaning device  6 M, all of which are arranged around drum-shaped photo-conductor  1 M.  
         [0034]     Image forming section  10 C, to form cyan toner images, includes charging device  2 C, exposure device  3 C, developing device  4 C, first transfer roller  5 C and cleaning device  6 C, all of which are arranged around drum-shaped photo-conductor  1 C.  
         [0035]     Image forming section  10 K, to form black toner images, includes charging device  2 K, exposure device  3 K, developing device  4 K, first transfer roller  5 K and cleaning device  6 K, all of which are arranged around drum-shaped photo-conductor  1 K.  
         [0036]     Intermediate transfer device unit  7  is structured of intermediate transfer device  70 , being an endless belt which rotates as shown with an arrow in  FIG. 1 , and rollers  71 ,  72 ,  73  and  74 . Intermediate transfer device  70  is an image carrier for carrying toner images which are to be transferred onto transfer material S. Image forming sections  10 Y,  10 M,  10 C and  10 K are image forming devices for forming toner images onto intermediate transfer device  70 .  
         [0037]     Unicolor toner images, each formed by image forming sections  10 Y,  10 M,  10 C and  10 K are sequentially transferred and overlapped onto rotating intermediate transfer device  70  by transfer rollers  5 Y,  5 M,  5 C and  5 K, and thereby multicolored toner image is created. Transfer material S, such as recording sheets, stored in sheet supplying cassette  20 , are supplied by sheet supplying rollers  21 , and passes through intermediate conveyance rollers  22 A,  22 B,  22 C and  22 D, and registration member  23 , after which transfer material S is conveyed to secondary transfer roller  5 A, serving as a transfer device, and multicolored toner image is together transferred onto transfer material S. Transfer material S, carrying the multicolored toner image is fixed by fixing device  24 , and then transfer material S is nipped and exhausted to tray  30  by exhausting rollers  25 .  
         [0038]     After the multicolored toner image is transferred onto transfer material S by secondary transfer roller  5 A, transfer material S is separated from intermediate transfer device  70 , and remaining toner is removed from intermediate transfer device  70  by cleaning device  6 A.  
         [0039]     During the image forming process, first transfer roller  5 K always presses intermediate transfer device  70  toward photo-conductor  1 K, but the other first transfer rollers  5 Y,  5 M and  5 C press intermediate transfer device  70  toward photo-conductors  1 Y,  1 M and  1 C, respectively, only when color image is being formed. Secondary transfer roller  5 A comes into pressure contact with intermediate transfer device  70 , when the secondary transfer is conducted onto transfer material S.  
         [0040]     Biased voltage, applied to first transfer rollers  5 Y,  5 M,  5 C and  5 K, as well as biased voltage, applied to secondary transfer roller  5 A, is monitored by a control section, which is not illustrated, and the conditions of the first and secondary transfer rollers are varied based on the monitored voltages so that the adequate transfer conditions are maintained.  
         [0041]     Volume resistivity of intermediate transfer device  70  is preferably 1×10 7 -1×10 11  Ω·cm, which can accomplish a stable image formation. Backup roller  74  is in pressure contact with secondary transfer roller  5 A through intermediate transfer device  70 . Each roller has a semi-conductive rubber surface, which is coated on a metallic shaft. For backup roller  74 , a semi-conductive rubber material having the volume resistivity of 1×10 5 -1×10 10  Ω·cm is preferably used, while for secondary transfer roller  5 A, a semi-conductive rubber material having the volume resistivity of 1×10 5 -1×10 10  Ω·cm is preferably used.  
         [0042]     Transfer materials S, stored in sheet supplying tray  20 , are picked up one by one by paired sheet supplying rollers  21 , then each sheet S is conveyed by paired intermediate conveyance rollers  22 A- 22 C to be nipped by registration member  23 .  
         [0043]     In order to transfer the images, carried on intermediate transfer device  70 , onto a predetermined position on transfer material S, registration member  23  works in synchronization with image formation, and conveys transfer material S to a transfer position where image transferring is conducted by secondary transfer roller  5 A. Numeral  26  represents a changeover gate to change conveyance paths, through one of which transfer material S is directly conveyed from fixing device  24  to sheet ejection roller  25 , and through the other of which transfer material S is downwardly conveyed toward reversible conveyance section  27 .  
         [0044]     Numeral  28  represents a reversing section to reverse transfer material S to enable formation of images on the reverse surface, numeral  29  represents a sheet re-supplying section to supply sheet S again for a reverse surface image formation.  
         [0045]     Transfer material S, ejected from fixing device  24 , is further ejected onto tray  30  by paired ejection rollers  25 , or conveyed to reversible conveyance section  27 .  
         [0046]     When transfer material S is to be flipped over and then ejected, firstly transfer material S is conveyed downward by reversible conveyance section  27 , next transfer material S is switched back and conveyed upward by reversible conveyance section  27 , after which transfer material S is ejected to tray  30  via paired ejection rollers  25 .  
         [0047]     When the images are to be formed on both sides of transfer material S, firstly transfer material S is conveyed downward by reversible conveyance section  27 , next transfer material S is switched back to be flipped over, then conveyed to sheet re-supplying section  29 , being a sheet re-supplying device, after which transfer material S is supplied to registration members  23  from sheet re-supplying section  29 .  
         [0048]     Then transfer material S is conveyed from registration member  23  to the transfer position where secondary transfer roller  5 A functions, where the toner images are transferred onto the reverse surface of transfer material S, and after fixing process is conducted for the images by fixing device  24 , transfer material S is ejected onto tray  30  by paired ejection rollers  25 .  
         [0049]      FIG. 2  shows conveyance path R from intermediate conveyance rollers  22 D ( 22  D 1  and  22 D 2 ) to transfer position P 2  via registration members  23  (rollers  231  and  232 ).  
         [0050]     Intermediate conveyance roller  22 D includes paired drive rollers  22 D 1  and  22 D 2 , while registration member  23  includes drive roller  231  and driven roller  232 . Drive roller  231  is structured of a core made of a rigid body, such as stainless steel, and a rubber layer made of a rubber, such as an ethylene propylene rubber, formed on the core. Driven roller  232  is formed of a rigid body such as stainless steel. A conveyance section located between intermediate conveyance roller  22 D and registration member  23  is formed of paired guide plates E 1  and E 2 .  
         [0051]     Conveyance path R, located between registration member  23  and transfer position P 2 , is formed of paired guide plates F 1  and F 2 , as well as paired guide plates G 1  and G 2 .  
         [0052]     A part of conveyance path R, which is adjacent to transfer position P 2 , is formed of intermediate transfer device  70 .  
         [0053]     Transfer material S is supplied to registration member  23  by intermediate conveyance rollers  22 D, and temporarily stopped by registration member  23 . During the temporary stop, transfer material S is looped in a wide section between guide plates E 1  and E 2 , where transfer material S is postured so that the top of transfer material S becomes perpendicular to the conveyance direction.  
         [0054]     After the temporary stop, registration member  23  is activated to convey transfer material S toward transfer position P 2 .  
         [0055]     As shown in  FIG. 3 , registration member  23  is operated in the timing related to exposure devices  3 Y,  3 M,  3 C and  3 K.  
         [0056]      FIG. 3  shows the operating timing charts of exposure devices  3 Y,  3 M,  3 C and  3 K, and registration member  23 .  
         [0057]     Exposure device  3 Y starts operation at time ta 1 , and stops at time tb 1 , forming images on a photo-conductor  1 Y. In the same manner, exposure device  3 M starts operation at time ta 2 , and stops at time tb 2 , exposure device  3 C starts operation at time ta 3 , and stops at time tb 3 , and exposure device  3 K starts operation at time ta 4 , and stops at time tb 4 .  
         [0058]     Registration member  23  starts operation at time ta 5 , and stops at time tb 5 . In addition, after registration member  23  stops at time tb 5 , a one way clutch, connected to registration member  23 , is deactivated, enabling registration member  23  to rotate freely following transfer material S.  
         [0059]     Based on the above-described timing control, image G, having peripheral areas of a predetermined width, is formed on transfer material S.  
         [0060]     Times ta 1 , ta 2 , ta 3  and ta 4  are determined by calculation based on the travel rate of intermediate transfer device  70  and distances among exposure devices  3 Y,  3 M,  3 C and  3 K.  
         [0061]     On the other hand, time ta 5  is determined by calculation based on the distance between an exposure position of exposure device  3 Y and transfer position P 2 , the travel rate of intermediate transfer device  70 , the distance between position P 1  of registration member  23  and transfer position P 2 , and the travel rate of transfer material S.  
         [0062]     Among the parameters used for the determination of times ta 1 -ta 5 , the travel rate of transfer material S is determined by the conveyance rate of registration member  23 , however, the conveyance rate of registration member  23  varies depending upon temperature.  
         [0063]     Drive roller  231  of registration member  23 , shown in  FIG. 5 , is structured of core RA formed of a stainless steel core which is covered with rubber layer RB. Coefficient of thermal expansion of rubber layer RB is so great that diameter D of drive roller  231  can vary by temperature change, which results in the change of the conveyance rate of drive roller  231 , whereby time ta 5  in  FIG. 3  changes due to changes in temperature.  
         [0064]     If the image forming apparatus is controlled excluding a countermeasure of the change of time ta 5 , generated is the change of clearance H at the top of transfer material S, shown in  FIG. 4 .  
         [0065]     The inventor of the present invention experimented the deviation of the top position of the image formed on transfer material S, to see how deviation was affected by diameter D of drive roller  231  of registration member  23 , thickness “d” of rubber layer RB, and conveyance distance L between position P 1  of registration member  231  and transfer position P 2 .  
         [0066]     In  FIG. 2 , position P 1  of registration member  23  is a point where line Q 1 , joining the centers of rotation of drive roller  231  and the center of rotation of driven roller  232 , intersects with conveyance path R. Next, transfer position P 2  is a point where line Q 2 , joining the center of rotation of transfer roller  5 A and the center of rotation of backup roller  74 , intersects with conveyance path R. Conveyance distance L between position P 1  of registration member  231  and transfer position P 2  is the length of conveyance path R in which transfer material S runs through guide plates F 1  and F 2 , and G 1  and G 2 , as shown by a dashed line.  
         [0067]     The inventor studied by the experiment that after image formation started during continuous image formation, the temperature of the area adjacent to registration member  23  increased, and when the temperature became greater by 20° C. than the starting temperature, the temperature came to equilibrium.  
         [0068]     The positional deviation of the images, printed on the front page and the reverse page, is actually disadvantageous.  
         [0069]     In  FIG. 6 , deviation Z was studied in such a way that with allowable value of 1.4 mm for deviation Z, generated between the top positions of the front image HG and the reverse page image RG, while parameters D, d and L were changed, the temperature of the area adjacent to registration member  23  was also changed during continuous image formation. In addition, the allowable value is not visually detectable by human eyes.  
         [0070]      FIG. 7  and Table 1 show the experimental results.  
                                                         TABLE 1                                       L [mm]                        50   70   110   170   210                       d/D   0.02       r.D   r.D   r.D   r.D               0.04               r.A   r.A               0.10           r.A       r.B               0.15               r.B   r.C               0.20           r.B   r.C               0.30   r.A               0.40       r.B   r.C                        
         [0071]     In  FIG. 7 , curve CV 1  is plotted with points which barely deviate with the best results, while Curve CV 2  is plotted with points which satisfy the allowable value of 1.4 mm, however with some amount of deviation.  
         [0072]     Curve CV 1  is shown by the following formula: 
 
 d/D= 103.901919 L   −1.493453  
 
         [0073]     The approximate formula of curve CV 1  is: 
 
 d/D= 103.9 L   −1.5   (1) 
 
         [0074]     Curve CV 2  is shown by the following formula: 
 
 d/D= 57.627133 L   −1.180607  
 
         [0075]     The approximate formula of curve CV 2  is: 
 
 d/D= 57.6 L   −1.2   (2) 
 
         [0076]     Previous Table 1 shows the experimental results based on the fundamental data of curves CV 1  and CV 2  in  FIG. 7 . In Table 1, “r.A” means the best results, while “r.B” means better results satisfying the allowable value of 1.4 mm, and “r.C” shows unacceptable results in which deviation is beyond the allowable value. The thickness of rubber layer to exhibit reasonable durability must be greater than or equal to 0.5 mm.  
         [0077]     “r.D” in Table 1 represents unacceptable results for durability due to a thinner rubber layer.  
         [0078]     By the above experiment, the registration member which does not change due to temperature at time ta 5  can be obtained, whereby correction control based on temperature at time ta 5  is not necessary, and images can be formed on transfer material S with higher positional accuracy.  
         [0079]      FIG. 8  shows the relevant parts of another example of the embodiments of the present invention.  
         [0080]     In  FIG. 8 , numeral  1  represents a drum-shaped photosensitive device as the image carrier, numeral  2  represents a charging device structured of a corona charging device, numeral  4  represents a developing device, numeral  7  represents a transfer device structured of a corona charging device, numeral  31  represents a separating device structured of a corona charging device, symbols E 1 -E 4  represent guide plates to guide transfer material P, and numeral  23  represents a registration member.  
         [0081]     Electrostatic latent images are generated on photosensitive device  1  by the charging operation of charging device  2  and the exposure operation of exposure device  3 , then the electrostatic latent images are developed by developing device  4 , whereby toner images are formed. In this manner, an image forming section is structured of charging device  2 , exposure device  3  and developing device  4 .  
         [0082]     Transfer material S is guided by guide plates E 1 -E 4 , conveyed by registration member  23 , and passes through transfer position P 2 , then the toner images, formed on photosensitive device  1 , are transferred onto transfer material S by transfer device  7 . Transfer material S, carrying the toner images, is separated from photosensitive device  1  by separating device  31 , and is conveyed to a fixing device, which is not illustrated, where the image is fixed.  
         [0083]     In this embodiment, registration member  23  includes drive roller  231  and driven roller  232 . Drive roller  231  is structured of a core made of a rigid body such as stainless steel, and a rubber layer formed on the core. A transfer section can be structured in such a way that formulas (1) and (2) can be established among diameter D of drive roller  231 , thickness d of the rubber layer, and conveyance distance L between position P 1  of registration member  23  and transfer position P 2 , whereby an image forming apparatus having less deviation of the top position of the images can be structured.  
         [0084]     Additionally, in the present example, transfer position P 2  being an end point of conveyance distance L is a point where straight line Q 3 , joining the center of rotation of photosensitive device  1  and charging electrode  7   a  of transfer device  7 , intersects with conveyance path R for transfer material S.  
         [0085]     Position P 1  of registration member  23 , to form another end point of conveyance distance L, is a point where line Q 1 , joining the center of rotation of drive roller  231  and the center of rotation of driven roller  232 , intersects with conveyance path R, the same as the first example.  
         [0086]     According to the present embodiments, this image forming apparatus enables the leading edge of the image to more accurately position on the transfer material, and further the images can be formed at lower production cost with higher accuracy as in the case of off-set printing.  
         [0087]     Furthermore, by satisfying formula (3) 
 
 d/D≦ 103.9 L   −1.5   (3) 
 
         [0088]     an image forming apparatus enables higher positional accuracy of printed images.  
         [0089]     Moreover, in double sided image formation of the preset embodiments, effectively prevented is the problem in which the position of an image on the front side differs from that of the reverse surface