Drum Cleaning Process and Apparatus for Electrophotography

The present invention relates to a drum cleaning process and apparatus for electrostatic photography by which residual toner particles are throughly removed from a photoconductive element after a transfer operation.

In a well known dry type electrophotographic process, a photoconductive drum or belt is electrostatically charged, radiated with a light image to cause local photoconduction and produce an electrostatic image, developed by applying a toner substance thereto to form a toner image and subjected to a transfer operation in which the toner image is transferred to a copy sheet. In practical application, it is impossible to transfer the toner image to the copy sheet completely, and residual toner remains on the drum after the transfer operation.

In order to utilize the drum for sucessive copying operations of different original documents, it is necessary to remove this residual toner from the drum, since double printing would occur if it were not removed.

In a wet electrophotographic process, a scraper blade is used to remove the residual toner. However, this expedient is not applicable to a dry electrophotographic process since abrasion of the delicate photoconductive surface layer on the drum would occur due to dry friction.

In order to remove the residual toner, it is necessary to dissipate the electrostatic image on the drum which strongly binds the residual toner to the drum surface. For this reason, most conventional dry electrophotographic machines comprise a corona discharge unit to apply an electric field opposite in polarity to the initial electrostatic charge to neutralize the electrostatic image on the drum, and a light source to subsequently illuminate the drum surface to cause photoconduction and further discharge of the drum. Since the electrostatic force between the residual toner and drum is reduced substantially, the residual toner may be more readily removed from the drum by a magnetic brush or the like.

A drawback exists in this prior art expedient in that the corona discharge is performed in the absence of light. The conductivity of the photoconductive layer in the dark is high, and it is impossible to dissipate the electrostatic charge in the lowermost portion of the layer. Even the subsequent exposure to light does not produce complete discharge, and the problem of double printing is not effectively solved.

It is an object of the present invention to provide a process for completely removing residual toner from a photoconductive member of an electrophotographic apparatus after a toner image transfer operation.

It is another object of the present invention to provide an electrophotographic apparatus embodying the process.

It is another object of the present invention to provide an electrophotographic process in which an electrostatic attractive force between residual toner particles and a photoconductive drum is dissipated through a simultaneous illumination of the drum surface with light and exposure of the drum surface to an electric field thereby effecting complete discharge of the drum and toner.

It is another object of the present invention to provide an electrophotographic process in which complete discharge of a photoconductive drum is produced by simultaneous illumination of the drum surface by light and exposure of the drum surface to an electric field, the effect of said steps in combination being greater than the aggregate effect of said steps performed sequentially.

It is another object of the present invention to provide an electrophotographic apparatus in which a magnetic brush which is used to apply toner substance to a photoconductive drum for development is also used to remove residual toner from the drum after a discharge operation comprising the simultaneous steps of illuminating the drum surface with light and exposing the drum to an electric field.

It is another object of the present invention to provide an electrophotographic apparatus in which a photoconductive drum is rotated adjacent to a light source and a corona discharge unit which simultaneously operate on the drum to dissipate an electrostatic charge thereon, the area of radiation of light on the drum surface overlapping the area of the electric field on the drum surface and extending downstream of the area of the electric field in the direction of rotation of the drum.

It is another object of the present invention to provide a generally improved electrophotographic process and apparatus embodying the process.

Other objects, together with the foregoing, are attained in the embodiments of the present invention described in the following description and shown in the accompanying drawings.

FIG. 1 is a schematic view of an electrophotographic apparatus embodying the present invention;

FIG. 2 is an enlarged schematic view illustrating an important aspect of the present invention; and

FIGS. 3 to 5 are fragmentary schematic views illustrating respective modifications of the present electrophotographic apparatus.

While the electrophotographic apparatus of the invention is susceptible of numerous physical embodiments, depending upon the environment and requirements of use, substantial numbers of the herein shown and described embodiments have been made, tested and used, and all have performed in an eminently satisfactory manner.

Referring now to FIG. 1, an electrophotographic apparatus 11 embodying the present invention comprises an electrically grounded rotary drum 12 formed with a photoconductive dielectric surface layer (not designated) and which is supported by a drive shaft 13. Drive means (not shown) rotate the shaft 13 and thereby the drum 12 clockwise at constant speed. A corona charging unit 14 is disposed adjacent to the periphery of the drum 12 and operative to electrostatically charge the surface of the drum 12 in the absence of light. A light source 16 is arranged to illuminate an original document 17 which is moved by a scanning system (not shown) in the direction of an arrow 18 at a speed equal to the tangential speed of the surface of the drum 12 in such a manner that an imaging system, shown in simplified form as a converging lens 19, radiates a light image of the document 17 onto the surface of the drum 12. The highlight areas of the light image cause the photoconductive layer of the drum 12 to conduct thereby dissipating the electrostatic charge in these areas and forming an electrostatic image of the document 17. A developing unit 21 which will be described in detail below applies a powdered toner substance to the drum 12 which is electrostatically attracted to the areas of the photoconductive layer in which the electrostatic charge is high, which correspond to the dark or background areas of the document 17. The developing unit 21 thereby forms a toner image of the document 17 on the drum 12.

At a transfer unit 22 comprising a corona charging unit 23 similar to the corona charging unit 14, a copy sheet 24 is fed in synchronized contact with the drum 12 and an electrostatic force is applied to the back of the sheet 24 thereby attracting and effecting transfer of the toner image to the sheet 24. A fixing unit (not shown) subsequently fixes the toner image by heat, pressure or both to the copy sheet 24. A discharge unit 26 then completely discharges the drum 12 and any residual toner is removed by the developing unit 21 as will be described in detail below.

The developing unit 21 comprises a hopper 31 which is filled with a powdered developing or toner substance preferably in the form of tiny granules or particles. Each granule comprises at least one particle of black carbon and at least one particle of iron, said particles being rigidly held together by a resin binder. A magnetic brush 32 is disposed closely adjacent to the surface of the drum 12 below an opening in the hopper 31 and comprises a magnetic core 33 surrounded by a non-magnetic sleeve 34, the core 33 and sleeve 34 being coextensive with the length of the drum 12. Either one or both of the core 33 and sleeve 34 are rotated so that toner from the hopper 31 is magnetically attracted to the magnetic brush 32 to form an accumulation of toner particles on the surface of the sleeve 34 and rotates in the direction of an arrow 36 which brushingly engages with the surface of the drum 12. A doctor 37 is provided at the opening of the hopper 31 to limit the thickness of the toner substance on the sleeve 34 to a required value. Due to the brushing engagement of the toner on the magnetic brush 32 with the surface of the drum 12, the toner is applied to the drum 12 to effect development of the electrostatic image into the toner image. The magnetic force of the magnetic core 33 is selected so that it is less than the electrostatic attraction between the electrostatic image on the drum 12 so that the toner may be transferred from the magnetic brush 32 to the drum 12 for development.

The discharge unit 26 comprises a housing 41 which acts as a light and electrical shield, the housing 41 being opaque, electrically conductive and electrically grounded. The housing 41 is coextensive with the length of the drum 12 and is formed with an opening 42 facing the drum 12. A light source 43 such as a fluorescent bulb is disposed in the housing 41 such that light from the light source 43 is radiated onto the surface of the drum 12 through the opening 42. A corona discharge electrode 44 is further disposed in the housing 41 such that an electric field of opposite polarity to that of the charging unit 14 is applied to the surface of the drum 12 through the opening 42.

As shown in FIG. 2, the housing 41 comprises, adjacent to the opening 42, an upstream wall 41a and a downstream wall 41b, the designations upstream and downstream referring to the direction of movement of the surface of the drum 12. The end of the downstream wall 41b adjacent to the drum 12 is cut away to define an opening 41c, and an electrically conductive transparent plate 41d formed of, for example NESA (trademark) glass, is fitted in the opening 41c.

In FIG. 2 the photoconductive surface layer of the drum 12 is designated as 12a and toner particles attracted to the surface layer 12a are shown as circles but are not designated by reference numerals. Due to the electrical shielding effect of the walls 41a and 41b, the electric field from the corona discharge electrode 44 is applied to the layer 12a in an area bounded by lines A and D as indicated by dashed lines leading from the electrode 44 to the layer 12a. The light from the light source 43 is radiated onto the layer 12a in an area bounded by lines B and E as indicated by dot-dash lines leading from the light source 43 to the layer 12a. It will be noted that the light passes through the plate 41d whereas the electric field is blocked or shielded thereby.

The light from the light source 43 and the electric field produced by the corona discharge electrode 44 are superimposed on the layer 12a in an area bounded by the lines B and D such that the layer 12a is simultaneously irradiated by light and electrically discharged in this position. The polarity of the corona discharge electrode 44 is opposite to that of the corona charging unit 14 so that the electrostatic image on the drum 12 is neutralized. The light causes photoconduction in the layer 12a so that any remaining charge on the layer 12a is shorted to ground due to the high conductivity of the layer 12a. In this manner, the charge on both the layer 12a and toner particles is completely removed, a result which cannot be achieved by corona discharge followed by exposure to light.

Assuming that the charge applied to the layer 12a by the charging unit 14 is negative in polarity, the polarity of the discharge electrode 44 is positive. The negative charge at the surface of the layer 12a produces electrostatic polarization of the toner particles so that they are positive at their ends next to the layer 12a and negative at their ends away from the layer 12a, thereby being electrostatically attracted to the layer 12a. A similar magnetic phenomenon attracts the toner particles to the magnetic brush 32 of the developing unit 31.

As viewed in FIG. 2, the electric field of the electrode 44 extends further upstream than the light from the light source 43, such that only the electric field exists in an area bounded by the lines A and B. This is of no practical consequence, and the housing 41 may be designed so that the electric field and light extend the same distance upstream or the light extends further upstream than the electric field. What is important, however, is that the electric field does not extend further downstream than the light.

If the plate 41d were opaque, the light from the light source 43 would be confined to an area on the layer 12a between the line B and a line C, as indicated by a solid line 46 leading from the light source 43 to the layer 12a, and only the electric field would exist in an area bounded by the lines C and D. As a result, the layer 12a would be electrostatically charged with a positive polarity, thereby inducing electrostatic polarization in the toner particles opposite to the normal polarity. The toner particles would be caused to be attracted to the layer 12a by the action of the discharge electrode 44, thereby defeating the purpose of the discharge unit 26. Since the toner particles would be attracted to the layer 12a as a result of the operation of the discharge electrode 44, it would be impossible for the magnetic brush 32 to completely remove the residual toner particles from the drum 12. However, due to the provision of the plate 41d, the light extends further downstream than the electric field and the problem of reverse charging of the layer 12a and toner particles is eliminated.

After operation on the drum 12 by the discharge unit 26, the magnetic brush 32 removes the residual toner particles from the drum 12 through physical contact. Since the electrostatic attractive force between the drum 12 and toner particles is reduced to substantially zero by the discharge unit 26, the magnetic attractive force of the magnetic brush 32 is effective in effecting complete transfer of the residual toner particles to the magnetic brush 32.

For each copying operation, the drum 12 is rotated through two revolutions. During the first revolution the copying operation is performed and the discharge unit 26 operates on the drum 12. During the second revolution, the charging unit 14 and light source 16 are de-energized so that the drum 12 is operated on by the magnetic brush 32 for removal of the residual toner after operation thereon by the discharge unit 26. Of course, if a second magnetic brush (not shown) is provided between the discharge unit 26 and charging unit 14, the entire copying and drum cleaning operation may be performed during one revolution of the drum 12, with the magnetic brush 32 serving only the developing function.

FIGS. 3 to 5 illustrate alternative forms of the housing 41 which provide the same function. In these Figures, a solid line 50 indicates the downstream limit of radiation of the light from the light source 43 and a dashed line 60 indicates the downstream limit of the electric field from the discharge electrode 44. Since the light source 43 and discharge electrode 44 are identical in all forms, the same reference numerals are utilized.

In FIG. 3 a housing 51 comprises an upstream wall 51a and a downstream wall 51b which have formed therebetween an opening 51c facing the drum 12. The downstream wall 51b is formed with an opening 51d in which is fitted an electrically conductive screen 51e. The screen 51e is substantially transparent to light but acts as a shield to the electric field, and serves the same function as the plate 41d.

In FIG. 4 a housing 61 comprises an upstream wall 61a and a downstream wall 61b which have formed therebetween an opening 61c facing the drum 12. The downstream wall 61b is perforated and bent outwardly to define an opening 61d which allows light to extend further downstream from the housing 61 than the electric field as indicated.

FIG. 5 shows a housing 71 which differs in construction from the housing 41 and comprises an upstream wall 71a and a downstream wall 71b. Whereas in the housing 41 the discharge electrode 44 is disposed between the light source 43 and the drum 12, in the housing 71 the light source 43 is arranged downstream of the discharge electrode 44. A partition wall 71c is provided in the housing 71 between the light source 43 and discharge electrode 44 and cut away at its bottom end to define an opening 71d. The walls 71a and 71c have formed therebetween an opening 71e facing the drum 12 and the walls 71b and 71c have formed therebetween an opening 71f facing the drum 12. The opening 71d is formed in such a manner that the electric field extends into the opening 71f but the light extends further downstream than the electric field.

Many modifications of the preferred embodiments shown and described within the scope of the invention will become possible for those skilled in the art after receiving the teaching of the present disclosure. For example, the invention is practically operable if the discharge electrode 44 is adapted to apply an alternating electric field to the drum 12 rather an electric field of opposite polarity to that of the charging unit 14.