Device and method for reducing reverse transfer of electrophotographic image

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.

CLAIM OF PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.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.sub.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.

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.sub.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.sub.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.sub.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.