Electrostatic latent image transferring apparatus
United States Patent Kurokawa et al. v  
 ELECTROSTATIC LATENT IMAGE TRANSFERRING APPARATUS Inventors: Junji Kurokawa; Susumu Tatumi,  
 both of Tokyo, Japan Assignee: Ricoh Co., Ltd., Tokyo, Japan Filed: Dec. 7, 1973 Appl. No.: 422,792  
 Foreign Application Priority Data Dec. l5, 1972 Japan 47-124188 US. Cl. 355/3 R; 355/17 Int. Cl 603g 15/00 Field of Search 355/3 R, 17; 96/l.4, 1.5;  
 References Cited UNITED STATES PATENTS 9/1964 Schaffert 355/17 X 5/l972 Arneth et al 96/l.5 X  
 Primary E.\&#39;aminerSamuel S. Matthews Assistant Examiner-Kenneth C. Hutchison Attorney, Agent, or Firm-Cooper. Dunham, Clark, Griffin &amp; Moran [57] ABSTRACT An electrophotographic apparatus for transferring electrostatic latent images comprising a photosensitive drum consisting of a photosensitive layer on a conductive body; a conductive transferring roller disposed in contact with the photosensitive layer for the purpose of transferring an electrostatic latent image formed on the photosensitive layer &#39;to an interposable electrostatic recording paper; and a varistor having one terminal electrically connected with the conductive body and the outer terminal grounded through the chassis of the apparatus. The transferring roller is also grounded through the chassis of the apparatus and the photosensitive drum is insulated relative to the chassis of the apparatus.  
 5 Claims, 3 Drawing Figures ELECTROSTATIC LATENT IMAGE TRANSFERRING APPARATUS BACKGROUND OF THE INVENTION a. Field of the Invention The present invention relates to an electrostatic latent image transferring apparatus for use in electrophotographic copying presses.  
 b. Description of the Prior Art The electrostatic latent image transferring method through which a copied image is obtained by transferring an electrostatic latent image formed on the photosensitive layer of an electrophotographic drum, through corona discharge and subsequent image exposure, onto an electrostatic recording paper consisting of a conductive paper support and a dielectric layer provided thereon, and then developing the thus transferred latent image has various merits, for example, inasmuch as the processes of developing and cleaning of the photosensitive layer on the drum are not required, the size of the copying press can be minimized and the life of the photosensitive layer can be prolonged. Nevertheless, this method has not been readily adaptable to practical use because the process of transferring therein in particular poses problems. As regards such an electrostatic latent image transferring method, Japanese Patent Publication No. 7115/1957 has disclosed a method of applying a corona discharge while pressing the support side of the recording paper with a transferring roller charged with bias voltage at the time of transferring. Actual incorporation of this method in the concrete form of an apparatus in a copying press, however, has proved to be attended with various problems.  
 That is, inasmuch as a corona discharge is to be applied 4 to the support side of the recording paper, there is a requirement of added space for the transferring section, unevenness of transference is apt to occur, and so on. Moreover, impression of a bias voltage on the transferring roller while pressing the roller against the support side of the recording paper is attended with a great deal of leakage current because the support side of the recording paper is generally conductive, so that it is infeasible to obtain a satisfactory transferred image in proportion to the costly power source required therefor. Japanese Patent Publication No. 19757/1967 has disclosed a method of transferring the electrostatic latent image by closely contacting the surface of the photosensitive layer carrying an electrostatic latent image with the surface of the dielectric layer of the recording paper and using merely the electric field to be produced on this occasion. This method is admittedly simple and capable of producing a distinct image, yet it is undesirable because in order to raise the potential for the transfer to the recording paper, a high-potential electrostatic latent image must be formed on the photosensitive layer and this will necessitate repeated use of the photosensitive layer for charging to a high potential by corona discharge, entailing remarkable shortening of the electrical life of the photosensitive layer.  
  The present invention is particularly directed to improving the apparatus for use in practicing the latter of the afore-cited methods (namely. the method disclosed in Japanese Patent Publication No. 19757/1967). That is, taking notice of the fact that the application ofa potential of less than 500V to the photosensitive layer in transferring an electrostatic latent image brings on practically no transfer on the electrostatic recording paper (transfer potential: 0V), the present invention is intended to provide an electrostatic latent image transferring apparatus which is simple and well suited for practical use and devised to improve the transferring efficiency without shortening the life of the photosensitive layer by charging to an optional potential of less than 500V on the conductive body of the photosensitive drum utilizing corona discharge current and raising the apparent surface potential of the photosensitive drum.  
 SUMMARY OF THE INVENTION The present invention involves an electrostatic latent image transferring apparatus for use in electrophotography, which comprises a photosensitive drum consisting of a photosensitive layer on a conductive body; a conductive transferring roller disposed in contact with the photosensitive layer for the purpose of transferring an electrostatic latent image formed on the photosensitive layer to an interposable electrostatic recording paper; and a varistor having one terminal electrically connected with the conductive body and the other terminal grounded through the chassis of the apparatus. The transferring roller is also grounded through the chassis of the apparatus and the photosensitive drum is selectively insulated relative to the chassis of the apparatus. The present invention may also include the use, with an electrostatic recording paper consisting of a conductive paper support and a dielectric layer, of at least one set of paper feed rollers disposed in front of and one set disposed to the rear of the transferring rollers for the purpose of supplying the electrostatic recording paper in between said transferring roller and photosensitive layer. The rollers disposed on the side of the dielectric layer of the recording paper consist of an insulating material while the rollers disposed on the side of the paper support consist of a conductive material and are grounded through the chassis of the apparatus.  
  The chassis of the apparatus&#34; herein means a device supporting the photosensitive drum, transferring roller, feed rollers, a low-voltage DC power source and the varistor, and may further include the frame, electrifier, optical system, developing means, fixing means and the like.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of an electrostatic latent image transferring apparatus embodying the present invention.  
  FIG. 2 and FIG. 3 show curves of the potential on the photosensitive layer of the drum illustrative of the effect of the apparatus under the present invention and illustrative of the effect of the conventional apparatus, respectively.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the photosensitive drum 1 is shown consisting of a photosensitive layer 11 and a conductive body 12. In the prior art, this photosensitive drum has been grounded through the chassis 10 of the apparatus, but in the present invention, the conductive body 12 of the photosensitive drum is selectively electrically insulated relative to the chassis 10 of the apparatus by means of a varistor 7. Transferring roller 2 not only operates as one electrode (disposed on the side of the conductive paper support 32) for the photosensitive drum at the time of transferring the image but also operates to improve the contact between the surface of the dielectric layer 31 of the electrostatic recording paper 3 and the surface of the photosensitive layer 11 [f image transference is performed employing the apparatus shown in FIG. 1 according to the prior art and electrifying the photosensitive layer 11 to the extent of 750V and 1,000V, respectively, by corona discharge in of the drum 1. Therefore, as the material for the trans- 5 the same way as above except for grounding the photoferring roller 2, it is preferable to employ an elastic sensitive drum 1 (that is, short-circuiting it directly to conductor such as a conductive rubber. An optical systhe conductive transferring roller 2 without employing tem 4, a corona discharger 5, and a high-voltage power the varistor 7), as expressed by the curves (1) and (2) source6connected with said corona discharger are apin FIG. 3, the effect of image transference in these propriately disposed about the drum 1. The conductive 10 cases as compared with the effect in the case of the apbody 12 of the photosensitive drum is held at a fixed paratus under the present invention employing the vapotential during corona discharge by virtue of the varisristor, is shown in the following table.  
 corona width of change voltage actpotential condition life of voltage slit for in ing on phototransferred of photosensiexposure potential sensitive to image image tive layer layer (frequency (KV) (mm) (V) (V) of use) conventional FIG. 3 low more than apparatus 5 .6 2 l 750 55 density 50,000 (grounded times drum) FIG. 3 up to -6.5 5 (2) 1000 100 satis- 5,000 factory times present apparatus (varistor 5.8 5 FIG. 2 750 l00 satismore than employing factory 50,000  
  250V times element) tor 7. Accordingly, application of the corona discharge More precisely, according to the prior art, in the case onto the photosensitive drum must be continued until completion of transference. Further, the apparatus shown in FIG. 1 can be provided with the abovementioned sets of paper feed rollers, the upper rollers 8a, 8b consisting of an insulating material such as resin, rubber or the like, while the grounded lower rollers 8c,  
 8d consist of a conductive material such as metal or the like, and all are appropriately disposed for the purpose of holding an electrostatic recording paper 3 therebetween and sending it through the transferring section and on to a developing section (not shown herein).  
  &#39;FIG. 2 is a graph showing the change in potential of the photosensitive drum in the case where image transference is performed employing an apparatus in accordance with the present invention. The photosensitive drum, provided with a photosensitive layer consisting of an organic semiconductor film of about 10p. in thick-. ness and having a diameter of 70 mm, is rotated at a ve-.  
 locity of 40 rpm, while employing a varistor capable of producing a voltage of about 250V when applying electric current of more than ;.LA, and setting the voltage to be impressed by corona discharge at 5.8KV. Halogen lamp light is applied by adjusting the width of a slit so as to attain an amount of exposure to produce 700 luxes in the intensity of illumination on the surface of the photosensitive layer. As seen in FIG. 2, simultaneously with the start of corona discharge, a quick rise in potential up to 250V takes place. This is the range of the potential on the conductive body 12 brought on by the varistor 7, and the range beyond this corresponds to the potential acting on the photosensitive layer brought on by the electric charge on the surface of the photosensitive layer. In this case, the maximum potential is 750V. Moreover, the potential at the time of transferring after exposure is 980V for the image area and 500V for the non-image area, and the potential transferred to the electrostatic recording paper is 100V for the image area and 0V for the nonimage area. Consequently, a satisfactory image can be produced upon development.  
 of FIG. 3(1) where the voltage acting on the photosensitive layer is set so as to make the electric charge on its surface, supplied by corona discharge, equivalent to that in the case of the present invention, the life of the photosensitive layer will equal that of the present invention but the transfer potential is low and only an image with low density can be obtained. In the case of FIG. 3 (2) where the photosensitive layer is charged with a high potential by corona discharge so as to make the transfer potential equivalent to that of the present invention, the transferred image is satisfactory, but repeated use thereof will give rise to partial dielectric breakdown of the photosensitive layer, and an increase of spots deprived of electric charge with the frequency of use of the photosensitive drum and lowering of the quality of image, thereby making it unfit for more than 50,000 times of use.  
  As discussed in the foregoing, according to the present invention, the apparatus is simple in construction and renders it possible to enhance the transferring effect as well as the durability of the photosensitive layer concurrently through mere application of an inexpensive varistor. As the applicable varistor, there are such commercial varistors as the zinc oxide varistor ZNR (voltage-dependent resistor element) manufactured by MATSUSHITA DENKl K.K. and the symmetric Zener diode manufactured by ISHIZUKA DENSHI K.K., but any constant voltage diode will do as long as it is capable of providing a fixed voltage as required when an electric current of more than SOuA is applied. As the photosensitive material to be employed for the photosensitive layer in the present invention, in addition to organic semiconductors, any photosensitive material applicable to general electrophotography will&#39;do. However, it goes without saying&#39;that, in the case of the apparatus shown in FIG. l,&#39;the polarity of the electricity charged by corona&#39;discharge varies-with the kind of photosensitive material employed? The following is an example of an embodiment of the present invention.  
 Example An electrostatic latent image was formed on a photosensitive drum consisting of a photosensitive layer of selenium having a thickness of 25p. and a metal body having a diameter of 70 mm, using a series-connected couple of zinc oxide varistors ERZ-O8D3K221 (220V element) manufactured by MATSUSl-llTA DENKI K.K. as the varistor and applying the conditions of a 40 r.p.m. velocity of rotation of the photosensitive drum, a +7.0 KV voltage impressed by corona discharge and 3 mm in the width of slit for exposure. The latent image was transferred to a commercial electrostatic recording paper and then developed by the conventional method. The apparent maximum voltage of the photosensitive drum on this occasion was 124OV, the maximum voltage acting on the selenium layer was 800V, the maximum potential on the body of the drum was 440V (in the case of an inorganic photoconductor such as selenium, the curve of light decay is sharp and the residual potential is not much as compared with an organic photoconductor, so that it can be used by raising the potential up to this extent), 1 10V in the transfer potential, and the resulting transferred image was satisfactory. The durability of the photosensitive selenium layer in this state amounted to satisfactory use as many as 100,000 times. Besides, the electrophotographic properties thereof after such extended use showed little change.  
 What is claimed is:  
  1. An electrostatic latent image transferring apparatus for use in electrophotography, comprising: a photosensitive drum consisting of a photosensitive layer on a conductive body; a conductive transferring roller disposed in contact with said photosensitive layer for transferring an electrostatic latent image formed on said photosensitive layer to an interposable electrostatic recording paper consisting of an image-receiving dielectric layer on a conductive paper support; a chassis of the apparatus for supporting said drum and said roller; and a varistor having one terminal electrically connected with said conductive body and the other terminal grounded through the chassis of the apparatus, said transferring roller being grounded through the chassis of the apparatus and said photosensitive drum being insulated relative to the chassis of the apparatus.  
  2. An apparatus as in claim 1, wherein said photosensitive layer comprises selenium.  
  3. An apparatus as in claim 1, wherein said transferring roller comprises a conductive rubber.  
  4. An apparatus as in claim 1, further comprising at least one set of paper feed rollers disposed in front of and one set disposed to the rear of said transferring roller, each set having one roller of an insulating material and one roller of a conductive material, said insulating feed rollers coming in contact with the dielectric layer side of said recording paper and said conductive feed rollers coming in contact with the paper support side of said recording paper and being grounded.  
  5. An apparatus as in claim 1 wherein said varistor comprises a constant voltage diode for producing a fixed voltage upon the application of currents greater than 50p, amps.