Method and apparatus for removing residual image from photoconductive element of electrophotographic copying machine

Abstract

A grounded roller is used to preclean the photoconductive element of a residual image prior to final cleaning by the same magnetic brush which was used in the development step. The density of residual image toner particles resulting from lack of transfer in areas of the electrostatic image on the photoconductive element due to undersized paper or the like is reduced to the density of the toner particles of the normal residual image areas by the grounded roller.

Description

The present invention relates to a method and apparatus for removing a residual image in the form of toner particles from a photoconductive drum of an electrophotographic copying machine.

In the type of electrophotographic copying machine to which the present invention is applied, a photoconductive drum is charged by means such as a corona discharge tube. The drum is then rotated into a position in which an optical image of a document being copied is projected thereon. Bright areas of the optical image cause the drum to locally conduct and dissipate the charge thereon to form an electrostatic pattern on the drum analogous to the optical image. A developing step is then performed in which charged toner particles are applied to the drum by a magnetic brush and adhere to the areas of the surface of the drum in which an electrostatic charge remains. The particles are then transferred to a sheet of copy paper and fixed thereto by heat or other means.

The present invention applies to this copy process after the transferring step. Since it is impossible to transfer all the toner particles to the copy paper, some will remain in the form of a residual image. If the residual image is not removed from the drum, it will be transferred to the next copy sheet in the form of undesirable double printing.

In the above described copying machine, the steps of charging, imaging, transferring, and a subsequent step of discharging the drum are performed during one revolution of the drum. During a second revolution of the drum, the charging, transfer and discharging units are de-energized, and a voltage is applied to the magnetic brush which is opposite in polarity to the charge of the charged toner particles. The residual toner particles are thereby removed from the drum by the magnetic brush. Although this cleaning method requires two complete revolutions of the drum to make one copy, it is preferable to prior art cleaning methods using feather brushes or scrapers. In the former case, toner particles will adhere to the feather brush and accumulate. The cleaning efficiency of the feather brush thus becomes very low, and the accumulated toner particles must be manually removed which is troublesome for the copying machine operator. The major disadvantage with a scraper is that it has a tendency to damage the sensitive surface of the photoconductive drum. Thus, the advantages of using a single magnetic brush for both developing and cleaning of the drum will be clearly understood, especially since the cleaning efficiency of the magnetic brush is much higher than a feather brush or a scraper and the toner particles removed from the drum can be automatically recycled into the developing tank.

However, in severe cases, not even a magnetic brush is sufficient to completely clean the drum. Prior art efforts to increase the cleaning effectiveness of a magnetic brush include the application of an extremely high voltage to the brush to attract the toner particles from the drum. This method, however, requires the use of an expensive high voltage source and may damage the surface of the drum by perforating the photoconductive layer thereof or other means.

Another problem exists in a case in which some toner particles do not contact the copy paper. An example is the use of an undersized copy paper which is smaller than the electrostatic image on the drum. In this case, the density of the toner particles in the outer areas of the drum which do not contact the copy paper will not be at all reduced, and will be much greater than the density of the normal residual toner particles in the inner areas of the drum which do contact the copy paper. It is especially difficult to remove toner particles from these outer areas of the drum, especially since during the transfer step, a voltage is applied to the copy paper which is opposite to the polarity of the charge of the toner particles. The charge of the toner particles in the outer areas of the drum will thereby be reversed. It is extremely difficult or impossible to completely remove toner particles from the drum in these outer areas using a magnetic brush alone, even if a high voltage is applied to the magnetic brush.

It is therefore an important object of the present invention to provide a method of removing residual toner particles from the photoconductive drum of an electrophotographic copying machine to an extent at which double printing is practically eliminated.

It is another object of the present invention to provide a method of removing residual toner particles from the photoconductive drum of an electrophotographic copying machine by which a high voltage is not required to be applied to the magnetic brush and a high voltage source is not required.

It is another important object of the present invention to provide a method of removing residual toner particles from the photoconductive drum of an electrophotographic copying machine by which both positively and negatively charged particles are effectively removed.

It is another important object of the present invention to provide a method of removing residual toner particles from the photoconductive drum of an electrophotographic copying machine by which toner particles which do not contact the copy paper during the transfer step are effectively removed.

It is still another important object of the present invention to provide an apparatus for carrying out the above mentioned method of removing residual toner particles from the photoconductive drum of an electrophotographic copying machine.

The above and other objects, features and advantages of the present invention will become clear from the following description taken with the accompanying drawings in which directions such as "clockwise" and "right" refer to the invention as viewed in the respective drawings and in which:

FIG. 1 is a side schematic view illustrating a method and apparatus according to the present invention;

FIGS. 2a, 2b, 2c, 2d and 2e illustrate steps in the method of FIG. 1;

FIG. 3 is a fragmentary side schematic view illustrating a modification of the method of FIG. 1; and

FIG. 4 is a fragmentary side view illustrating another modification of the method of FIG. 1.

Referring now to FIG. 1, an apparatus embodying a method according to the present invention for removing residual toner particles from a photoconductive drum 10 is illustrated, in which the drum 10 is fixed to a shaft 12 through which it is rotated by drive means (not shown). The drum 10 is grounded through the shaft 12, and has a photoconductive layer 14 formed on its surface. The nature of the layer 14 is such that its electrical resistance decreases in proportion to the intensity of light incident thereon. An optical system 16 is arranged to project an optical image of an original document 18 to be copied onto the surface of the drum 10. A charging unit 20 is arranged adjacent to the layer 14 of the drum 10 and comprises a corona discharge element 22 in a reflector or shield 24. A developing unit 26 includes a developing tank 28 housing a magnetic brush 30 which is mounted by means of a conductive roller on a shaft and is rotatable as shown by an arrow by driving means (not shown). A voltage may be applied to the magnetic brush 30 by means of a voltage source 32. The developing tank 28 contains a developing mixture consisting of charged toner particles and a carrier such as iron particles which is applied onto the surface of the layer 14 of the drum 10 by the magnetic brush 30.

A transfer unit 34 is identical in construction to the charging unit 20 and comprises a corona discharge element 36 and a shield 38. A sheet of copy paper 40 is fed between the drum 10 and transfer unit 34 with its upper surface in close contact with the layer 14 by feed means (not shown) in the direction indicated by an arrow. After passing through the transfer unit 34, the paper 40 passes through a fixing unit (not shown).

A conductive roller 42 is maintained in physical rolling contact with the drum 10 by a shaft 44, and the roller 42 is electrically grounded through the shaft 44. If desired, the roller 42 may be arranged to protrude from a tank 46 supporting a brush or wiper 48 in sliding contact with the roller 42, although the tank 46 and wiper 48 may be omitted if desired.

A discharge unit 50 is identical in construction to the charging unit 20 and comprises a corona discharge element 52 and a shield 54. The corona discharge elements 22 and 36 of the charging unit 20 and the transfer unit 34 respectively are connected to a positive voltage source (not shown), and the corona discharge element 52 of the discharge unit 50 is connected to a negative voltage source (not shown).

The operation of the invention will now be described.

As the drum 10 is rotated by the drive means, the surface of the layer 14 is charged to a positive polarity by the charging unit 20 at a point A. Since the drum 10 has not yet been exposed to light, the layer 14 will have low electrical conductivity and the positive charge will be retained thereon. When the drum 10 rotates to a position B, an optical image of the surface of the document 18 will be projected thereonto by the optical system 16. The arrangement is such that an electrostatic image will be formed on the surface of the layer 14 because the conductivity of the layer 14 will increase in proportion to the intensity of light incident thereon. For example, in a bright area, the conductivity of the layer 14 will decrease tremendously, and the charge in the bright area will be shorted to ground through the layer 14, the drum 10 and the shaft 12. In a dark area, however, the conductivity of the layer 14 will remain low, and the positive charge will remain on the surface of the layer 14.

The drum 10 is then rotated to a position C, at which toner particles from the tank 28 are applied onto the surface of the layer 14 by the magnetic brush 30. The toner particles are arranged to have a negative charge, and the carrier particles may be neutral or have a positive charge. Since the toner particles have a negative charge, they are attracted to the positively charged areas of the layer 14 of the drum 10 and adhere thereto to form a toner image.

The drum 10 is then rotated to a position D, at which the paper 40 is fed in physical contact therewith by the feed means. As the drum 10 and paper 40 move together in surface to surface contact, the toner image is transferred to the paper 40. This operation is ensured by the transfer unit 34, which applies a positive potential to the bottom surface of the paper 40. The negatively charged toner particles are attracted to the upper surface of the paper 40 by the positive potential of the transfer unit 34, and adhere to the paper 40. The paper 40 is then fed through the fixing means in which the toner image is thermally or otherwise fixed to the paper 40.

The drum 10 is then advanced to a position E, at which it rolls in contact with the roller 42. Since the roller 42 is grounded, toner particles are attracted and adhere thereto, and are thereby removed from the drum 10. Toner particles carried on the surface of the roller 42 are discharged to ground therethrough, and are removed therefrom by the wiper 48 and drop down into the tank 46 for recycling. However, not all of the toner particles will be removed by the roller 42, and the drum 10 will rotate to a position F adjacent to the discharging unit 50 with toner particles still adhered thereto.

Since the polarity of the electromotive force of the discharging unit 50 applied to the drum 10 is negative, the original positive charge on the surface of the layer 14 will neutralized and the electrostatic image will be erased from the surface of the layer 14. The toner particles will then electrically float on the surface of the layer 14. The drum 10 will then rotate past the position A and begin another complete revolution.

During the second revolution, the charging unit 20, transfer unit 34 and discharging unit 50 are de-energized. During the first revolution, no voltage was applied to the magnetic brush 30, but during the second revolution, a positive voltage is applied thereto by the source 32. The optical system 16 is also inoperative. As the drum 10 rotates to the position C, the negatively charged floating toner particles are removed from the drum 10 by a combination of physical contact and the positive charge on the magnetic brush 30. After the final cleaning operation is completed, the toner particles will automatically be deposited in the tank 28 for recycling.

This operation will now be examined in more detail to distinctly point out the advantages of the present invention with reference to FIGS. 2.

FIG. 2a illustrates the process when the drum 10 is between the positions C and D, in which toner particles 100 have been deposited onto and are adhering to the layer 14.

In FIG. 2b, the drum 10 is at the position D, and a positive potential is applied to the top of the paper 40 through the transfer unit 34 by a source 60. It will be assumed that the paper 40 is smaller than the toner image on the layer 14, and that toner particles 100a of the toner image will contact the paper 40 and toner particles 100b of the image will not contact the paper 40. In this case, some upper toner particles 100a will be transferred to the paper 40 to provide a visual copy of the document 18, and some lower toner particles 100a will remain adhered to the layer 14 of the drum 10 as a residual image but will retain their negative charge. The toner particles 100b, on the other hand, will all remain adhered to the layer 14 and will furthermore be exposed directly to the positive potential of the transfer unit 34. The charge of the toner particles 100b will thereby be converted to positive.

It will be understood from the above description that all of the toner particles 100b cannot be removed from the drum 10 by the magnetic brush 30 alone, since their density is much greater than that of the toner particles 100a. The present invention provides novel means in the form of the roller 42 to remove toner particles 100b from the layer 14 to the extent that their density will be reduced to approximately that of the toner particles 100a. This is possible since the roller 42 is grounded and will attract positive toner particles 100b as well as negative toner particles 100a as shown in FIG. 2c, in which the drum 10 is in the position E in contact with the roller 42.

As shown in FIG. 2d, the drum 10 has rotated past the position E to the position F, in which it is adjacent to the discharging unit 50. A negative potential is applied to the transfer unit 50 by a source 70, which discharges the layer 14 and converts the polarity of the charge of the toner particles 100b to negative. When the drum 10 has rotated through the positions A and B to the position C, all toner particles 100a and 100b will be of negative polarity and will be removed from the layer 14 of the drum 10 by the positively charged magnetic brush 30 in a manner as shown in FIG. 2e.

In order to enhance the effectiveness of the invention, the capacitance C.sub.2 of the roller 42 may be higher than the capacitance C.sub.1 of the drum 10. The resistivity of the roller 42 may be a selected value between 10.sup.6 .OMEGA.cm and 10.sup.10 .OMEGA.cm.

FIG. 3 shows a modification of the present invention, in which the roller 42 is disposed inside of or above the tank 28 and is in brushing contact with the magnetic brush 30. The wiper 48 is held in sliding contact with the roller 42 but it may be dispensed with if preferred. As shown, the magnetic brush 30 is able to remove toner particles from both the layer 14 of the drum 10 and from the roller 42, and the removed toner particles may be removed from the magnetic brush 30 by a modified wiper 48' and drop into the tank 28 for recycling.

In FIG. 4, a further modification of the present invention is useful in cases in which a magnetic toner is used. A source 80 applies a voltage opposite in polarity to that of the toner particles, in this case positive, to a corona discharge unit 82 comprising a corona discharge element 84 and a shield 86, and thereby to a dielectric layer 42' formed on the surface of the roller 42. The efficiency of removing toner particles from the layer 14 of the drum 10 is thereby increased. If desired, the source 80 may apply a negative potential to the layer 42' in order to attract positive toner particles 100b rather than negative toner particles 100a.

Regarding the embodiment of FIG. 3, it has been determined experimentally that if the roller 42 is so disposed that the first cleaning step is performed after the discharging step is completed, it will be easier to remove the toner particles 100b and harder to remove the toner particles 100a. However, all forms of the present invention herein shown and described have been experimentally proven to be workable and useful assuming that a proper balance between the potentials applied by the units 20, 34, 50 and 82. In understanding the present invention, it might be useful to consider the transfer of the usable toner image onto the paper 40 at the position D as a first transfer operation, and the transfer of the residual image to the roller 42 a second transfer operation. Using this concept, it will be realized that the second transfer operation can be performed either between the first transfer step at the position D and the discharging step at the position F, or between the discharging step at the position F and the final removal or cleaing step at the position C during the second revolution of the drum 10 with the minor variation in performance as described above.

Claims

What is claimed is:

1. An apparatus for removing a residual image from a photoconductive member of an electrophotographic copying machine after the photoconductive member has been utilized in the steps of sequentially charging, imaging, developing and transferring, the apparatus comprising:

a grounded member to partially remove the residual image from the photoconductive member through physical contact therewith;

a charged member to remove the remainder of the residual image from the photoconductive member through physical contact therewith, and

a source of electromotive force to discharge the photoconductive member after performing the transferring step,

said photoconductive member being a roller, and in which the grounded member is disposed between the source and the charged member in the direction of rotation of the photoconductive member.

2. An apparatus according to claim 1, in which the charged member is a roller in rolling contact with the photoconductive member, the charged member is a magnetic brush, and the magnetic brush in contact with both the photoconductive member and the grounded member to remove the residual image from both the photoconductive member and the grounded member.

3. An apparatus according to claim 2, in which the grounded member is formed with a dielectric layer on its surface, and which further comprises a source of electromotive force to charge the surface of the dielectric layer.

4. An apparatus according to claim 2, in which a wiper is held in sliding contact with the magnetic brush.

5. An apparatus according to claim 2, in which the grounded member and the magnetic brush are housed in a developing tank into which removed toner particles forming the residual image drop for recycling.

6. An apparatus according to claim 1, in which the grounded member is a roller in rolling contact with the photoconductive member, and further comprising a wiper in sliding contact with the roller.

7. An electrophotographic process comprising the steps of:

a. charging a rotating photoconductive drum to a first polarity;

b. applying a light image to the drum;

c. applying toner particles of a second polarity opposite to the first polarity to the drum with a magnetic brush to produce a toner image;

d. transferring the toner image utilizing an electric field of the first polarity;

e. pressing a grounded member against the drum;

f. charging the magnetic brush to the first polarity; and

g. pressing the magnetic brush against the drum; whereby toner particles adhere to the drum after step d as a residual toner image, some being charged to the first polarity during step d, a portion of the toner particles being removed by the grounded member during step e so that the densities of the residual toner particles of the first and second polarities are substantially equalized, the remainder of the residual toner particles being removed from the drum by the charged magnetic brush during step g.

8. A process according to claim 7, in which the magnetic brush is in a fixed location relative to the drum and is in constant frictional contact therewith, and in which step a is performed during one revolution of the drum and step g is performed during a second revolution of the drum.

9. A process according to claim 7, further comprising the step of:

h. applying an electric field of the second polarity to the drum after step e and before step g to convert the residual toner particles of the first polarity to the second polarity.

10. An apparatus for removing a residual image from a photoconductive member of an electrophotographic copying machine after the photoconductive member has been utilized in the steps of sequentially charging, imaging, developing and transferring, the apparatus comprising:

a grounded member to partially remove the residual image from the photoconductive member through physical contact therewith, the grounded member and the photoconductive member being rollers in rolling contact with each other; and

a charged member to subsequentially remove the remainder of the residual image from the photoconductive member through physical contact therewith, the charged member being a magnetic brush which is used in the developing step during one revolution of the photoconductive member and to remove the remainder of the residual image from the photoconductive member during a second revolution of the photoconductive member.

11. An apparatus according to claim 10, further comprising a source of electromotive force to discharge the photoconductive member after performing the transferring step.

12. An apparatus for removing a residual image from a photoconductive member of an electrophotographic copying machine after the photoconductive member has been utilized in the steps of sequentially charging, imaging, developing and transferring, the apparatus comprising:

a grounded member to partially remove the residual image from the photoconductive member through physical contact therewith;

a charged member to remove the remainder of the residual image from the photoconductive memver through physical contact therewith, and

a source of electromotive force to discharge the photoconductive member after performing the transferring step,

said photoconductive member being a roller, and in which the grounded member is disposed between the charged member and the source in the direction of rotation of the photoconductive member.

13. An apparatus according to claim 12, in which the grounded member is arranged to protrude from a tank in which means are supported in sliding contact with the grounded member to remove toner particles therefrom.

14. An apparatus for removing a residual image from a photoconductive member of an electrophotographic copying machine after the photoconductive member has been utilized in the steps of sequentially charging, imaging, developing and transferring, the apparatus comprising:

a grounded member to partially remove the residual image from the photoconductive member through physical contact therewith and in which the capacitance of the grounded member is greater than that of the photoconductive member; and

a charged member to remove the remainder of the residual image from the photoconductive member through physical contact therewith.

15. A method of removing charged particles from photoconductive member, comprising the steps of:

removing a portion of the charged particles having a charge of a first polarity from the photoconductive member through physical contact with a grounded member;

converting the first polarity of the charge of the remainder of the portion of the charged particles to a second polarity which is opposite to the first polarity so that the charge of all the particles is of the second polarity; and

removing the charge of all the particles of the second polarity through physical contact with a charged member having a charge of the first polarity.