Abstract:
A device and a method for reducing contamination of the rear surface of recording papers during formation of a next image, with residual developing material adhered to a photosensitive drum and a transfer roller, by polarizing the recording papers.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled Device And Method For Reducing Reverse Transfer of Electrophotographic Image earlier filed in the Korean Industrial Property Office on Sep. 2, 1996, and there duly assigned Serial No. 96-37923 by that Office. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrophotography developing systems. More specifically, the present invention relates to image transfer processes and mechanisms in an electrophotography developing system. 
     2. Description of the Prior Art 
     Electrophotography developing systems are widely used in copying machines, laser beam printers, LPH (LED(Light Emitting Diode) print head) printers, paper facsimile machines and the like. Conventional electrophotography developing entails repeating a cycle of charging a medium, exposing and developing the medium, then transferring the image to another medium and fixing it thereto. Contact charging systems have been adapted widely to minimize generation of ozone that occurs during the charging operation. 
     Several types of electrophotography apparatuses are described in the patent literature. U.S. Pat. No. 5,166,734, for an Imaging System Including Pre-transfer Discharge to Pinhas et al., employs a photoconductive drum which is charged by a corotron. The image portions are discharged while the background portions remain at full charge. Referring to column 4, lines 39-44, a spray of liquid toner may be directed onto a portion of the roller 38 or onto a portion of photoconductive drum 10. Referring to column 5, lines 63-68, an intermediate transfer member 30 is maintained at a voltage and temperature suitable for electrostatic transfer of the image thereto. Referring to column 7, lines 20-26, the optimum transfer potential of the intermediate transfer member is -400 volts. The image transfer medium does not contain the surface having a positive charge. 
     U.S. Pat. No. 5,140,375, for an Image Forming Apparatus to Shindo et al., includes a brush which induces a bias voltage on a recording sheet. Referring to column 7, lines 59-65, the bias voltage induced on the sheet is controlled between 0.25 to 2.0K Volts and exhibits the same polarity as the toner. 
     U.S. Pat. No. 5,036,360, for a Moisture Compensation for Electrostatographic Apparatus to Paxon et al., induces an electrostatic charge of a predetermined primary voltage V o  to the surface of a photoconductive belt. Referring to column 3, lines 26-30, the belt brings the areas bearing latent charge images into a development station 38. Referring to lines 33-38, charged toner particles on preselected magnetic brushes are attracted to the oppositely-charged latent image patterns on the belt. 
     I have noticed that the developing material that remains, that is, the non-transfer developing material, adheres to the photosensitive drum due to fatigue phenomenon common to photosensitive drums. This non-transferred developing material contaminates the next image to be transferred to the recording papers. Additionally, some of the non-transferred developing material is transferred to the transfer roll. Some of this residual developing material on the transfer roll subsequently is transferred to the rear surface of subsequent recording papers being developed, introducing unwanted latent images thereon. 
     I have found that the art represented by these exemplary references demonstrates a need for an electrophotography system which minimizes the contamination of recording papers with latent images due to the unwanted transfer residual developing material. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a device and process for reducing reverse transfer of an image forming apparatus using an electrophotography developing system, wherein a photosensitive drum adhered with developing material contacts a charged front surface of a recording paper, and a transfer roller contacts the rear surface of the recording papers, the rear surface exhibiting a charge having the same polarity as the developing material and the front surface exhibiting a charge having the opposite polarity. 
     An object of the invention is to provide a device and method for reducing contamination on the front and rear surfaces of recording papers by residual developing material adhered to the photosensitive drum and transfer roller of an electrophotography apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above ands other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of an electrophotography developing system; 
     FIG. 2 is a schematic diagram of an electrophotography developing system constructed according to the principles of the present invention; and 
     FIG. 3 is a diagram showing the charge dynamics of the photosensitive drum, transfer roller and recording paper during the practice of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a typical contact changing system is depicted schematically. A conductive roller or brush 14 with a uniform negative charge contacts a photosensitive drum 18 creating a potential on the surface of the photosensitive drum 18 in the range of about -500 volts to -600 volts. The reference character S represents the path of recording papers. The photosensitive drum 18 rotates in the direction indicated by the arrow, driven by an engine (not shown). 
     Simultaneously, a pair of conveyor rollers 10 convey recording papers from a cassette (not shown) to a pair of register rollers 12. The register rollers 12 order the leading edges of the recording papers prior to their introduction into the transfer roller 24. 
     An exposing device 16 exposes the surface of the photosensitive drum 18 corresponding to an image to be printed, thus creating an electrostatic latent image at the surface of the drum. The surface potential of -500 volts to -600 volts is maintained on the non-exposed portion of the drum while the electrostatic latent image portion has a surface potential of about 0 volts. 
     The electrostatic latent image at the surface of the photosensitive drum 18, then must be developed and converted into a visible image with developing material. A developing bias voltage V D  of about -450 volts is applied to a developing roller 22, thus the developing material applied thereto by a doctor blade 20 also exhibits a negative charge. The developing material is transferred to the exposed region of the photosensitive drum 18 by the potential difference of the developed region where the developing roller 22 and the photosensitive drum 18 contact each other. The developing material next is transferred from the exposed region of the photosensitive drum 18 to the recording papers. The transfer roller 24 draws the developing material from the surface of the photosensitive drum 18 toward the recording papers with a transfer voltage V T  of about 1.3 to 1.5 kilovolts. 
     Once the developing material is transferred to the recording papers, it must be fixed at the surface of the recording papers by heat and pressure of a heating roller 28 and a pressure roller 26. Once fixed, the recording papers are ejected to an exterior of the image forming apparatus to copy or print the recording papers. 
     Some developing material remains adhered to the photosensitive drum 18, hereinafter referred to as non-transfer developing material, due to fatigue phenomenon common to photosensitive drums. This non-transferred developing material contaminates the next image to be transferred to the recording papers. Additionally, some of the non-transferred developing material is transferred to the transfer roll. Some of this residual developing material on the transfer roll subsequently is transferred to the rear surface of subsequent recording papers being developed, introducing unwanted latent images thereon. 
     Referring to FIG. 2, a schematic diagram of the present electrophotography developing system is provided. The device includes a power supply V R  which imparts a bias voltage of -10 to -100 volts on conductive register rollers 32. The register rollers 32 convey and simultaneously apply the voltage potential to the rear surface of the recording papers, thus the front surface is polarized at 10 to 100 volts. 
     Referring also to FIG. 3, the charged recording papers are received in a developing area, between the transfer roller 24 and the photosensitive drum 18. The recording papers are charged such that the developing material 104 on the photosensitive drum 18 is conveyed to the front surface 100 of the recording papers only. Any residual reverse-charge developing material (not shown) on the surface of the photosensitive drum 18, having a positive potential, is discouraged from transferring the positively-charged front surface 100 of the recording papers. 
     The transfer roller 24, having received some residual developing material 106 from the photosensitive drum 18, ordinarily would re-transfer the material 106 to the rear surface 102 of recording papers 30. However, according to the present invention, the rear surfaces 102 of the recording papers are negatively charged, thus repel the retransfer of the residual developing material 106 from the transfer roller 24. 
     In a preferred embodiment of the present invention, the recording papers conveyed through the register rollers to the transfer roller and the photosensitive drum each have a rear surface that is charged negatively and a front surface that is charged positively. When the rear surface of the recording papers has a negative charge, and the residual developing material adhered to the transfer roller has a negative charge, less residual developing material is transferred to the rear surface of the recording papers. This occurs because like charges repel, thus the negatively charged developing material on the transfer roller repels the negatively charged rear surface of the recording papers. 
     Also, any reverse charge developing material remaining on the photosensitive drum, exhibiting a positive charge, is discouraged from transferring to the front surface of the recording papers, which is positively charged. This occurs because, although the photosensitive drum repels the reverse charge developing material, the repulsion is significantly less than that exerted by the recording papers. 
     As apparent from the foregoing, the present invention advantageously reduces the contamination of the rear surface of the recording papers to provide a cleaner image upon successive image transfers. 
     It should be understood that the present invention is not limited to the particular embodiment disclosed herein, rather to include all embodiments with in the scope of the appended claims.