In electrophotographic machines, copies of documents or other subjects are produced by creating an image of the subject on a photoreceptive surface, developing the image and then fusing the image to copy material. In some machines, the copy material may itself be specially prepared with a photosensitive coating enabling the image to be placed directly upon the copy material. In machines utilizing plain bond copy paper or other ordinary image receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays reflected from the document to be copied. Since white areas of the original document reflect large amounts of light, the photoreceptive material is discharged in white areas to relatively low levels while the dark areas continue to contain high voltage levels even after exposure. In that manner, the photoreceptive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. present on the original document.
After receiving the image, the photoreceptor is moved to a developing station where a developing material called toner is placed on the image. This material may be in the form of a black powder which carries a triboelectric charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, it adheres to the surface of the photoreceptor in proportions related to the shading of the original. Thus, black character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded half tone character portions of the original should receive intermediate amounts.
The developed image is moved from the developer to a transfer station where a copy receiving material, usually paper, is juxtaposed to the developed image on the photoreceptor. A charge is placed on the back side of the copy paper so that when the paper is stripped from the photoreceptor the toner material is held on the paper and removed from the photoreceptor. Unfortunately, the transfer operation seldom transfers 100% of the toner from the photoreceptor to the copy paper. Toner remaining on the photoreceptor after transfer is called "residual toner" and may amount to 15% or more of the toner present on the photoreceptor prior to transfer.
The remaining process steps call for permanently bonding the transferred toner material to the copy paper and cleaning the residual toner left on the photoreceptor so that it can be reused for a subsequent copy production.
In the cleaning step, it is customary to pass the photoreceptor under a preclean charge generating station to neutralize the charged areas on the photoreceptor. The photoreceptor may also be moved under an erase lamp to discharge any remaining charge. In that manner, the residual toner is no longer held by electrostatic attraction to the photoreceptive surface and thus it can be more easily removed at a cleaning station.
In order to avoid overburdening the cleaning station, it is customary to remove all charge present on the photoreceptive surface outside of the image area prior to the development step. This is usually done by using an interimage erase lamp to discharge photoreceptive material between the trailing edge of one image and the leading edge of the next. Also, erase lamps are used to erase charge along the edges of the photoreceptor outside of the image area. For example, if the original document is 8.5.times.11 inches in size, and if a full sized reproduction is desired, the dimensions of the image on the photoreceptor will also be 8.5.times.11 inches. The interimage and edge erase lamps remove charge outside of the 8.5.times.11 inch area.
The copy process above described has been in use for many years in the document copier industry. Machines utilizing this process have been well received by the general public since the quality of the copy produced is superior. However, electrophotographic machines of the transfer type are not without problems; one is that the machines are expensive and a second is that they are complex and have a tendency to break down. One of the major causes of these problems has been the cleaning apparatus used in the process. While many improvements have been made, no one has successfully provided a cleaning station which is completely reliable and, in fact, the cleaning station is usually one of the most troublesome components in commercial machines. Moreover, quality cleaning is difficult; that is, even after the cleaning operation, some residual toner is frequently left upon the photoreceptive surface. This residual toner can show up as high background on copies and also can build up and create a toner film on photoreceptive surfaces which ultimately destroys the value of the surface and necessitates the installation of a new surface. Additionally, since the cleaning station cleans away residual toner, it uses up the charge of toner in the developer thus creating a need for adding toner. Since toner is an expensive supply item, this need to replace toner adds to the cost of operating the machine. Finally, since the cleaning station is accumulating toner, service is periodically necessary simply to remove the toner from the cleaning station. To solve this problem, some machines have attempted to recycle cleaned away toner by sending it back to the developer station. Attempts to recycle toner, while a good idea, have usually brought added reliability problems to the machine to such an extent that recycling apparatus is not in common use.
To solve these problems, it has been reasoned that the best remedy is to simply rid the machine of the need for a cleaning station. To do that, a combined developer/cleaner apparatus was invented so that residual toner could be cleaned from the photoreceptive surface in the developer itself thus eliminating the need for additional troublesome apparatus and avoiding the accumulation of residual toner outside the developer. Cleaner/developer apparatus of this type is the subject of U.S. Pat. No. 3,647,293 which describes a two-cycle process where the development occurs on a first cycle and the cleaning occurs on a second cycle. While this two-cycle process is a suitable solution to the problems of separate cleaning stations, it is apparent that two cycles of the photoreceptor are needed to produce a single copy and therefore the technique is limited to slower speed machines. Considerable effort has been expended in attempting to remove a cleaning station from one-cycle machines so that the advantages of eliminating a cleaning station could be brought to higher speed equipment. For example, U.S. Pat. No. 3,649,262 describes a copier machine with cascade type developer/cleaner apparatus in a one-cycle operation. The patent suggests that there are a great many variables which must be considered to make the machine operate successfully. Included among these variables is the position of the development electrode relative to the vertical in the cascade type developer, the value of the charge voltage, the value of the development electrode voltage, the developer flow rate, the charge density of the original image, toner size, toner concentration, and carrier size. The techniques described in this patent can successfully produce a few copies without a separate cleaning operation; however, the quality of the copies rapidly deteriorate thus making the process more of a laboratory curiosity than one that can be successfully commercialized.
U.S. Pat. Nos. 3,628,950 and 3,640,707, assigned to the assignee of the 3,649,262 patent described above, expressly refer to difficulties in the process of the 3,649,262 patent and attempt to solve them. The process of the 3,628,950 patent appears essentially identical to the 3,649,262 patent but does contain some discussion of an additional variable, the preclean corona current level. The 3,640,707 patent supplies an extra electrode to the cascade developer in an attempt to aid in the removal of residual toner. Neither of these improvement attempts produced the techniques and solutions of the instant invention and neither have resulted in the production of a commercially successful one-cycle machine without a cleaning station.
U.S. Pat. Nos. 3,598,580 and 3,646,866 disclose a copier machine where a magnetic brush cleaner/developer apparatus is used in a one-cycle operation. These patents state that when organic photoconductors are used, there is an ability to transfer most of the powder image to the copy paper and thereby obviate the need for cleaning. Again, however, the quality of an image according to the process of these patents rapidly deteriorates as successive copies are produced as shown in FIGS. 19-22 herein where an organic photoconductor was used.
All of the above work was performed several years ago but the problem still remains; how can troublesome cleaning stations be removed from electrophotographic machines without using a two-cycle process.