Abstract:
There is described an image forming apparatus which forms a toner image on a transfer material by transferring a toner image from a photoreceptor element to an intermediate transfer member and then transferring the toner image from the intermediate transfer member to the transfer sheet. The apparatus includes: a developing section to develop the electrostatic latent image with toner so as to form the toner image; a first transferring section to transfer the toner image formed on the photoreceptor element onto the intermediate transfer element; a second transferring section to further transfer the toner image onto a transfer material; a lubricant supplying section to supply a lubricant, which includes at least a fatty acid calcium salt, onto the photoreceptor element in such a manner that the lubricant is added to the toner; and a polishing section to polish a surface of the intermediate transfer element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to an image forming apparatus which forms a toner image on a transfer material by transferring a toner image from a photoreceptor element, serving as a photosensitive member, to an intermediate transfer member (hereinafter, also referred to as a intermediate transfer element) and then transferring the toner image from the intermediate transfer member to the transfer sheet.  
         [0002]     To assure transferability and cleaning abilities of toners, a conventional image forming apparatus has used a developing agent made of toners and lubricants containing Ca- or Zn-related fatty acid metal salts.  
         [0003]     Patent Document 1: Japanese Non-Examined Patent Publication H05-188643.  
         [0004]     Patent Document 2: Japanese Non-Examined Patent Publication S60-131547.  
         [0005]     Further, the conventional art uses toners, which have a sharp size distribution like polymerized toner and are almost spherical without edges. As these toners are sharp in the size distribution, the charge distribution is also sharp in most cases and their developing properties are good. However, the toners will be easily packed closest, agglutinates, and hard to be transferred. Additionally, such toners are hard to be cleaned. Patent Documents 3 to 6 try to assure transferability and cleaning ability of toners by applying a lubricant to the photoreceptor element besides lubricants added to the toners.  
         [0006]     Patent Document 3: Japanese Non-Examined Patent Publication H06-148941.  
         [0007]     Patent Document 4: Japanese Non-Examined Patent Publication 2001-282043.  
         [0008]     Patent Document 5: Japanese Non-Examined Patent Publication H11-212398.  
         [0009]     Patent Document 6: Japanese Non-Examined Patent Publication H11-219087.  
         [0010]     However, if the lubricant applied to the photoreceptor element and the lubricant added to the toners are not proper, the photoreceptor element may have a lubricant film on the surface of the photoreceptor element.  
         [0011]     Patent Document 7: Japanese Non-Examined Patent Publication 2001-166659.  
         [0012]     The lubricant film on the surface of the photoreceptor element is acceptable as far as it is uniform on the whole surface of the photoreceptor element. However, when the photoreceptor element is cleaned by a cleaning blade or the like, some part of the lubricant film may locally remain un-removed on the surface of the photoreceptor element. As the lubricant is high in electric resistance, the total resistance of the local films left on the surface of the photoreceptor element may be high and as the result the transfer ratio reduces. This causes local longitudinal streaks of low or high densities in the image.  
         [0013]     Further, naturally, it is impossible to uniformly apply a lubricant to the surface of the photoreceptor element. The lubricant is locally excessive on some part of the surface of the photoreceptor element and insufficient on the other part of the surface. This makes the developing ability and the transferability uneven and finally makes the image rough. In an image forming apparatus that transfers a toner image from a photoreceptor element to an intermediate transfer member and then transfers a toner image from the intermediate transfer member to a transfer sheet, it is impossible to control the formation of a lubricant film on the intermediate transfer member. Consequently, in such an image forming apparatus, the image density may be uneven on the transfer sheet.  
         [0014]     Further, the image densities are made uneven also by deterioration of toners in the developing devices. This often occurs when images of low-print areas are formed continuously. This is because most of toners are circulated in the developing devices vainly. As the result, the lubricant added to the toners is lost and the developing ability, transferability, and cleaning ability of the toners are deteriorated.  
       SUMMARY OF THE INVENTION  
       [0015]     To overcome the abovementioned drawbacks in conventional image-forming apparatus, it is an object of the present invention to provide an image forming apparatus that forms images without any streak caused by uneven films on a photoreceptor element, serving as a photosensitive member, or an intermediate transfer element and without damaging the photoreceptor element, the intermediate transfer element, and the cleaning blade.  
         [0016]     Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by image-forming apparatus described as follow.  
         [0017]     (1) An apparatus for forming an image, comprising: a photoreceptor element; a charging section to charge the photoreceptor element; an exposing section to expose an electrostatic latent image on the photoreceptor element; a developing section to develop the electrostatic latent image with toner so as to form an toner image; an intermediate transfer element; a first transferring section to transfer the toner image formed on the photoreceptor element onto the intermediate transfer element; a second transferring section to further transfer the toner image, transferred onto the intermediate transfer element by the first transferring section, onto a transfer material; a lubricant supplying section to supply a lubricant, which includes at least a fatty acid calcium salt, onto the photoreceptor element in such a manner that the lubricant is added to the toner; and a polishing section to polish a surface of the intermediate transfer element.  
         [0018]     (2) The apparatus of item  1 , wherein an amount of the lubricant added to the toner is equal to or greater than 0.03%-by-mass of an amount of the toner.  
         [0019]     (3) The apparatus of item  1 , wherein an amount of the lubricant added to the toner is equal to or greater than 0.2%-by-mass of an amount of the toner.  
         [0020]     (4) The apparatus of item  1 , further comprising: a lubricant applying section to apply the lubricant onto the photoreceptor element.  
         [0021]     (5) The apparatus of item  1 , wherein the polishing section includes a brush roller having fibers in which a titanium oxide serving as an abrasive material is mingled, and polishes the surface of the intermediate transfer element in such a manner that the brush roller is driven to rotate in a state of directly contacting the intermediate transfer element.  
         [0022]     (6) The apparatus of item  1 , wherein a plurality of photoreceptor elements, each of which is equivalent to the photoreceptor element and corresponds to each of primary colors, and a plurality of developing sections, each of which is equivalent to the developing section and corresponds to each of the primary colors, are provided for forming a color image.  
         [0023]     (7) The apparatus of item  1 , further comprising: a cleaning section to clean the surface of the intermediate transfer element.  
         [0024]     According to the present invention, in the image forming apparatus having an intermediate transfer member, the formed images are free from streaks due to uneven films on the surface of the intermediate transfer member. Further, the image forming apparatus can form a lot of images steadily for a long time without curling and destroying the cleaning blade of the endless-belt shaped intermediate transfer member even if a serial multiple copy processing continues. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0026]      FIG. 1  shows the cross-sectional view of the configuration of a color image forming apparatus embodied in the present invention;  
         [0027]      FIG. 2  shows the cross-sectional view of a means for applying lubricant to the photoreceptor element and a polishing means of the image forming apparatus embodied in the present invention; and  
         [0028]      FIG. 3  shows the means for polishing the intermediate transfer member of the image forming apparatus embodied in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     This invention will be described in further detail by way of embodiments. It is to be expressly understood, however, that the description is not intended to limit the technical ranges and terms in claims. Further, the assertive explanation in the embodiments of this invention is for the best mode of this invention and is not intended to limit the technical ranges and terms of this invention.  
         [0030]      FIG. 1  shows the cross-sectional view of the configuration of a color image forming apparatus which is an embodiment of this invention.  
         [0031]     This image forming apparatus  100  is generically called a tandem type full-color image forming apparatus and consists of some sets of an image forming section  10  ( 10 Y,  10 M,  10 C, and  10 K), an endless belt-like intermediate transfer unit  7 , a paper delivery means  21 , and a fixing device  24 . A document image reader SC is provided on the top of the main body A of the image forming apparatus.  
         [0032]     The image forming section  10 Y for forming a yellow image consists of a drum-like photoreceptor element  1 Y which is surrounded by a charger  2 Y, an image exposing means  3 Y, a developer  4 Y, a primary transfer roller  5 Y as a primary transfer means, and a cleaning means  6 Y. The image forming section  10 M for forming a magenta image consists of a drum-like photoreceptor element  1 M which is surrounded by a charger  2 M, an image exposing means  3 M, a developer  4 M, a primary transfer roller  5 M as a primary transfer means, and a cleaning means  6 M. The image forming section  10 C for forming a cyan image consists of a drum-like photoreceptor element  1 C which is surrounded by a charger  2 C, an image exposing means  3 C, a developer  4 C, a primary transfer roller  5 C as a primary transfer means, and a cleaning means  6 C. The image forming section  10 K for forming a black image consists of a drum-like photoreceptor element  1 K which is surrounded by a charger  2 K, an image exposing means  3 K, a developer  4 K, a primary transfer roller  5 K as a primary transfer means, and a cleaning means  6 K.  
         [0033]     The endless belt-like intermediate transfer unit  7  as an intermediate transfer member is equipped with an endless belt-like intermediate transfer member  70  as a semi-conductive endless belt type intermediate transfer member which are supported and rotated by a plurality of rollers.  
         [0034]     Color images formed by the image forming sections ( 10 Y,  10 M,  10 C, and  10 K) are respectively transferred onto the moving endless belt-like intermediate transfer member  70  in sequence by the corresponding primary transfer rollers ( 5 Y,  5 M,  5 C, and  5 K) to form a multi-color image. A transfer sheet P such as a paper sheet as a recording medium is picked up from a paper cassette  20  by a paper feeding means  21 , carried by a plurality of intermediate rollers ( 22 A,  22 B,  22 C, and  22 D) and registration rollers  23 , then carried to the secondary transfer rollers  5 A as the secondary transfer means, and has all color images transferred onto the transfer sheet P at a time. The transfer sheet having the multi-color image on it is fixed by the fixing device  24  and ejected to the outside stack tray  26  by the ejection rollers  25 .  
         [0035]     After transferring a multi-color image onto the transfer sheet P by the secondary transfer roller  5 A as the secondary transfer means and separating the transfer sheet P in a curving manner, the endless belt-like intermediate transfer member  70  is cleaned to remove the left-over toner by the cleaning means  6 A.  
         [0036]     While an image is formed, the primary transfer roller  5 K is always pressed against the photoreceptor element  1 K. The other photoreceptor elements ( 1 Y,  1 M, and  1 C) are pressed against the corresponding transfer rollers ( 5 Y,  5 M, and  5 C) only when color images are formed.  
         [0037]     The secondary transfer roller  5 A is pressed against the endless belt-like intermediate transfer member  70  only when transferring a multi-color image onto the transfer sheet P that passes through these rollers  5 A.  
         [0038]     The assembly  8  can be pulled out from the main body A of the image forming apparatus by means of supporting rails ( 82 L and  82 R).  
         [0039]     The assembly  8  consists of the image forming sections ( 10 Y,  10 M,  10 C, and  10 K) and the endless belt-like intermediate transfer unit  7 .  
         [0040]     The image forming sections ( 10 Y,  10 M,  10 C, and  10 K) are vertically disposed in series. The endless belt-like intermediate transfer unit  7  is provided to the left of the photoreceptor elements ( 1 Y,  1 M,  1 C, and  1 K). The endless belt-like intermediate transfer unit  7  consists of the endless belt-like intermediate transfer member  70  that are supported and moved by rollers ( 71 ,  72 ,  73 ,  74 ,  76 , and  77 ), primary transfer rollers ( 5 Y,  5 M,  5 C, and  5 K), and the cleaning means  6 A.  
         [0041]     When pulled out of the main body of the image forming apparatus, the assembly  8  comes out together with the image forming sections ( 10 Y,  10 M,  10 C, and  10 K) and the endless belt-like intermediate transfer unit  7  in a body.  
         [0042]     The image forming apparatus forms a toner image on each photoreceptor element ( 1 Y,  1 M,  1 C, and  1 K) by charging, exposing, and developing, transfers each color image in sequence to form a multi-color image on the endless belt-like intermediate transfer member  70  (as a primary transfer), transfers a multi-color image onto the transfer sheet P at a time, presses and heats to fix the image on the transfer sheet P by the fixing device  24 . After transferring each color toner image onto the endless belt-like intermediate transfer member  70 , each photoreceptor element ( 1 Y,  1 M,  1 C, and  1 K) is cleaned to remove the left-over toner by the corresponding cleaning means ( 6 Y,  6 M,  6 C, and  6 K), and repeats the above charging, exposing, and developing steps to form the next image.  
         [0043]     Each photoreceptor element ( 1 Y,  1 M,  1 C, and  1 K) is 60 mm in the diameter and its process speed is 220 mm/s. The image forming apparatus of this invention is a tandem type full-color image forming apparatus  100  using an endless belt-like intermediate transfer member  70  as an intermediate transfer member. The primary transfer roller in the apparatus  100  is a 20 mm-diameter sponge roller whose resistance is 1×10 7  Ω and the primary transfer is controlled by a constant current control method.  
         [0044]     As shown also in  FIG. 2 , lubricant applying means ( 1 YP,  1 MP,  1 CP, and  1 KP) are provided to apply lubricant to the photoreceptor elements ( 1 Y,  1 M,  1 C, and  1 K). Each lubricant applying means is a 12 mm-diameter brush roller that plants conductive acrylic fibers of 6.25 deniers on the surface of the roller at a density of 100K fibers per square inch. The brush rotates opposite to the rotation of the photoreceptor element at a line speed of 132 mm/sec. Each lubricant ( 1 YS, MS,  1 CS, and  1 KS) is 8 mm wide×5 mm high×332 mm high and has the hardness equivalent to pencil hardness HB.  
         [0045]     The lubricant contains at least fatty acid metal salt of calcium stearate. The toner as a developing agent is a polymerization toner of 6.5 μm in size. The quantity of the lubricant is added at a concentration of 0.01 to 0.3%-by-mass to the total quantity of toner (of each color).  
         [0046]     Besides the cleaning means  6 A that removes the left-over toner from the surface of the endless belt-like intermediate transfer member  70 , a polishing means  7 G is provided to polish the surface of the endless belt-like intermediate transfer member  70 . This means  7 G is a brush roller that plants nylon fiber mixed up with titanium oxide. The brush is pressed at a bite of 1.0 mm against the intermediate transfer member and driven to rotate opposite to the rotation of the intermediate transfer member.  
         [0047]     The endless belt-like intermediate transfer member  70  is a single layer half-conductive polyimide belt of 0.1 mm thick×861 mm circumferential×362 mm wide and the surface resistivity of 10 10  to 10 11  Ω·cm.  
         [0048]     We inventors tested and evaluated image formation of the image forming apparatus of this invention as follows:  
         [0049]     Image Formation Test 1A:  
         [0050]     We made 200,000 copies by the image forming apparatus equipped with the polishing means  7 G in accordance with the above embodiment. (Test #1 to Test #10).  
         [0051]     Image Formation Test 1B:  
         [0052]     We made 200,000 copies by the image forming apparatus without the polishing means  7 G in accordance with the above embodiment. (Test #11 to Test #20).  
         [0053]     We checked the first copy and the 200,000 th  copy for longitudinal streaks in images, image roughness, and curling of the cleaning blade of the endless belt-like intermediate transfer unit and evaluated the images by the following: 
        Level 5 and 4: Excellent     Fair but not practical     Level 2 and 1: Poor and not acceptable.        
 
         [0057]     The test results are illustrated in Table 1.  
         [0058]     In all test, the first copies are free from longitudinal streaks (Level 5).  
                                                                                                             TABLE 1                                       Common evaluation           (1A and 1B)                    Curl-up                   of       1B: With a polisher       cleaning       for the intermediate   1A: Without a   blade for       transfer belt only   polisher   the                        Longitudinal           Longitudinal   intermediate           Test   Ca   streaks   Test   Ca   streaks   transfer   Toner       No.   concentration   in image   No.   concentration   in image   member   scattering                    1   0.01   5   11   0.01   5   1   1       2   0.02   5   12   0.02   5   2   1       3   0.03   5   13   0.03   3.5   4   1       4   0.04   5   14   0.04   2   4   1       5   0.05   5   15   0.05   2   5   1.5       6   0.06   5   16   0.06   1.5   5   2.5       7   0.08   5   17   0.08   2   5   3       8   0.1   5   18   0.1   2   5   3.5       9   0.2   5   19   0.2   2   5   4       10   0.3   5   20   0.3   2   5   4.5                  
 
         [0059]     Longitudinal streak level of image: level 4 or higher (acceptable).  
         [0060]     The starting copies are all excellent.  
         [0061]     Without a polisher, longitudinal streaks in images can be suppressed only in a very limited range of Ca concentration. However, in the concentration range, the cleaning blade is curled up badly. Further, toner scattering cannot be ignored.  
         [0062]     With a polisher for the intermediate transfer belt, the longitudinal streaks are eliminated strikingly (in comparison of the use of Zn-contained lubricant). This is a synergetic effect of the Ca-contained lubricant and a belt polisher.  
         [0063]     This cannot be expected from the conventional art.  
         [0064]     When the image forming apparatus has no polishing means  7 G for the intermediate transfer member, it is very hard to eliminate longitudinal streaks in copied images unless the concentration of the lubricant (%-by-mass) is low (see Test #11 and Test #12). With the polishing means  7 G, longitudinal streaks in copied images are suppressed strikingly in the wide range of concentration (%-by-mass) of the lubricant (see Test #1 to Test #10). However, when the concentration of the lubricant goes below 0.03%-by-mass, unwanted problems appear such as curl-up of the cleaning blade for the intermediate transfer member. Therefore, the concentration of the lubricant is preferably 0.03%-by-mass or more. The lubricant concentration of 0.2%-by-mass or more is more preferably to effectively suppress scattering of toner in the image forming apparatus (see Test #9 to Test #10).  
         [0065]     Test #2A, Test #2B, and Test #2C, use different kinds of lubricants. These tests are different from Test #1A, and Test #1B in that the lubricants are not calcium stearate but contain fatty acid metal salt of zinc stearate.  
         [0066]     The image formation tests were implemented as follows:  
         [0067]     Image Formation Test 2A:  
         [0068]     We made 200,000 copies by the image forming apparatus equipped with the polishing means  7 G only for the intermediate transfer member in accordance with the above embodiment.  
         [0069]     Image Formation Test 2B:  
         [0070]     We made 200,000 copies by the image forming apparatus without the polishing mean  7 G for the intermediate transfer member.  
         [0071]     Image Formation Test 2C:  
         [0072]     The image forming apparatus for this test is equipped with means for polishing the surfaces of photoreceptor elements ( 1 Y,  1 M,  1 C, and  1 K) besides means ( 6 Y,  6 M,  6 C, and  6 K) for cleaning the surfaces of photoreceptor elements ( 1 YG,  1 MG,  1 CG, and  1 KG). These polishing means ( 1 YG,  1 MG,  1 CG, and  1 KG) are prepared by dispersing powder of strontium titanate in the lubricant that is to be applied to the surfaces of photoreceptor elements ( 1 Y,  1 M,  1 C, and  1 K). In other words, we made 200,000 copies by the image forming apparatus by the image forming apparatus equipped with both the polishing means  7 G for the intermediate transfer members and the polishing means ( 1 YG,  1 MG,  1 CG, and  1 KG) for the photoreceptors elements.  
         [0073]     We used the same levels as those for Test #1A and Test #1B for evaluation.  
         [0074]     The test results are illustrated in Table 2.  
                                                                                                                                             TABLE 2                                       Common               2C: With   evaluation (2A,               polishers   2B, and 2C)            2A: With a       for the   Curl-up           polisher for the       intermediate   of       intermediate       transfer belt and   cleaning       transfer belt   2B: Without a   the photoreceptor   blade       only   polisher   elements   for the                    Longitudinal           Longitudinal           Longitudinal   intermediate   Toner       Test   Zn   streaks   Test   Zn   streaks   Test   Zn   streaks   transfer   scat-       No.   concentration   in image   No.   concentration   in image   No.   concentration   in image   member   tering                    31   0.01   5   41   0.01   5   21   0.01   4.5   1   1       32   0.02   5   42   0.02   5   22   0.02   5   2   1       33   0.03   4   43   0.03   3   23   0.03   5   4   1       34   0.04   3   44   0.04   1   24   0.04   5   4   1       35   0.05   3   45   0.05   1   25   0.05   5   5   1.5       36   0.06   3   46   0.06   1   26   0.06   4.5   5   2.5       37   0.08   3.5   47   0.08   1   27   0.08   5   5   3       38   0.1   3   48   0.1   1   28   0.1   5   5   3.5       39   0.2   3   49   0.2   1   29   0.2   5   5   4       40   0.3   3   50   0.3   1   30   0.3   5   5   4.5                  
 
         [0075]     The effect becomes greater as the polishing means are applied to the intermediate transfer belt and the photoreceptor elements.  
         [0076]     The effect of using the Zn-contained lubricant is not so dramatic as that of using the Ca-contained lubricant.  
         [0077]     →This is what we expected from the conventional art.  
         [0078]     As see from Table 2, it has been difficult to eliminate longitudinal streaks in copied images unless the concentration of the lubricant (%-by-mass) is low even when the polishing means  7 G is provided for the intermediate transfer member (see Test #31, Test #32, and Test #33). However, in this lubricant concentration range, the cleaning blade for the intermediate transfer member may be curled up or damaged.  
         [0079]     Contrarily it is found that, when the image forming apparatus is equipped both the belt polishing means  7 G and the polishing means for the photoreceptor elements, streaks in images are strikingly improved in the wide concentration range of lubricant.  
         [0080]     Judging from the test results of Table 1 and Table 2, when the lubricant contains fatty acid metal salt of calcium stearate containing Ca although the lubricant for the developing agents contains fatty acid metal salt of zinc stearate containing Zn, the polishing means for the intermediate transfer members only can strikingly eliminate longitudinal streaks in images in a very wide range of the lubricant concentration (%-by-mass). This enables the image forming apparatus to go without polishing of the photoreceptor elements, reducing the production cost of the apparatus and increasing the service lives of the photoreceptor elements.  
         [0081]     Disclosed embodiment can be varied by a skilled person without departing from the spirit and scope of the invention.