Electrophotographic Developing Process And Apparatus

Abstract

This invention relates to an electrophotographic process wherein first and second zones of a photoconductor are caused to pass a developing electrode, the first zone having an electrographic charge image thereon and the second zone being electrostatically charged but not having been exposed to produce an image thereon and the two zones passing the electrode in either order, and wherein the electrical field between the electrode and the photoconductor is less when the second zone is passing the electrode than it is when the first zone is passing the electrode, and further relates to an apparatus for carrying out this process.

Description

The present invention relates to an electrophotographic developing process and to apparatus for carrying out this process.

In electrophotographic copying machines in which the intermediate image carrier is a photoconductive drum or an endless photoconductive belt, only certain portions are exposed to produce an image, no exposure taking place outside these areas. This leads to the charge which is sprayed on being retained on the photoconductor in the areas which are not exposed for producing an image. Very large quantities of toner are applied to these unexposed areas of the photoconductor, especially when the developing system is suitable for developing not only zones where sudden increases in voltage occur, but also charged surfaces. This leads both to high consumption of toner and a heavy work-load at the adjacent cleaning station.

In an attempt to solve this problem, a proposal has been made to fit a quenching (discharging) lamp upstream of the developing station. This quenching lamp is always switched on when a zone of the photoconductor that has not been exposed for producing an image passes below the lamp. Illumination by means of such a lamp results in the charge flowing away, and developing of these unexposed zones of the photoconductor is avoided.

The above method, however, has considerable disadvantages. The circuit necessary for controlling the quenching lamp is complicated and the lamp uses up a relatively large amount of energy. The lamp constitutes an additional element in the electrophotographic cycle and thus increases the likelihood of the machine breaking down and, because of the expense involved in providing the fixing means and the lamp itself, increases the cost of the electrophotographic reproduction machine. Furthermore, the use of a quenching lamp imposes a considerable load upon the photoconductor.

The present invention provides an electrophotographic process wherein first and second zones of a photoconductor are caused to pass a developing electrode, the first zone having a latent electrographic charge image thereon and the second zone being electrostatically charged but not having been exposed to produce an image thereon and the two zones passing the electrode in either order, and wherein the electrical field between the electrode and the photoconductor is less when the second zone is passing the electrode than it is when the first zone is passing the electrode. The electric field when the second zone is passing the electrode is advantageously so much reduced compared with the field when the first zone is passing the electrode that there is either no electrical field or the field is in the opposite direction to the field when the first zone is passing the electrode. If the field is reversed there is no need for the numerical value of the reversed field to be less than the numerical value of the field when the first zone passes the electrode, although normally only a weak reversed field will be applied.

The present invention makes it possible to eliminate substantially the disadvantages of the prior art processes. In the process of the invention, those zones of the photoconductor that have been charged but are not exposed to produce an image use up little, if any, toner in the electrophotographic cycle, and no quenching lamp is required.

According to the invention, an opposing field can particularly advantageously be set up by applying a higher voltage to the developing electrode than the voltage applied when the first zone is passing the electrode. A voltage for preventing the developing of the residual charge that has not completely flowed away in the exposed zones may, if desired, by applied to the electrode when the first zone is passing it. This can be done by means of an additional voltage source or a voltage source having an adjustable E.M.F.

In the process of the invention, the electrode may comprise a magnet brush or be part of a cascade developing system. If a cascade developing system is used instead of a magnetic brush, the opposing electrical field can of course be set up only when that zone of the photoconductor that has been exposed in the manner for producing an image has been completely removed from the vicinity of the electrode.

The invention also provides apparatus for carrying out the process of the invention. The apparatus comprises a photoconductor capable of being electrostatically charged and on which an electrographic charged image can be formed, a magnetic brush, means connected to the brush for supplying to the brush either of two different voltages having the same sign but different absolute magnitudes, and switch means adapted to connect the lower voltage to the brush when a zone of the photoconductor which has been exposed to produce an image is passing the brush and to connect the higher voltage to the brush when no such zone is passing the brush. The apparatus may comprise one adjustable voltage source or two different voltage sources. In a modification of the apparatus described above, the magnetic brush may be replaced by the developing electrode of a cascade developing system.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic sectional view of an electrophotographic copying machine,

FIG. 2 shows a further diagrammatic sectional view of the copying machine shown in FIG. 1, some parts being omitted for clarity, and

FIG. 3 is a diagrammatic sectional view of an electrophotographic reproduction machine incorporating a cascade developing station.

Referring now to the drawings, FIG. 1 shows an electrophotographic reproduction machine which comprises a drum 1 with a photoconductor 2 fitted thereon. A potential V.sub.0 can be applied to the drum. The photoconductor may be charged in a uniform manner in a corona-charging station 3 and exposed to produce an image in an exposure station 4. The resultant charge pattern can be developed in a developing station 5, and the developed image can be transferred to a copy-receiving material 7, for example paper, in a transfer station 6. A magnetic brush 8, to which either of two potentials V.sub.1 and V.sub.2 may be applied is provided for the developing operation.

For various reasons it is not possible simply to pass the photoconductor as exposed for producing an image through the various stations of the electrophotographic cycle. Instead, zones of various sizes that have not been exposed for producing an image are present between those zones of the photoconductor that have been so exposed. In previously proposed electrophotographic reproduction machines, particularly in those with which charged surfaces can also be developed, the zone which is not exposed for producing an image is, like the zone which is exposed, charged in the charging station and developed in the developing station. Since this zone is not exposed, the resultant developing in the developing station is as though the zone were a black area in the image. Thus, not only is a relatively large quantity of toner used up in the previously proposed machines, but this toner has also to be removed from the photoconductor, and it rapidly stops up the filtering means.

In the arrangement illustrated in FIG. 1, that zone of the photoconductor between a and b is exposed for producing an image, whereas the zone between b and c is not so exposed. According to the invention, the developing of the zone b - c, that is not exposed for producing an image, is prevented in the following manner. A potential V.sub.1, which differs from the potential V.sub.0 applied to the drum, is applied in the normal manner to the magnetic brush 8 used for the developing operation when the zone a - b is passing beneath the brush. The purpose of this difference between V.sub.1 and V.sub.0 is to prevent developing of the "white" areas in the exposed zone a - b, which would otherwise occur as a result of the incomplete removal of charge in these zones. In order to prevent developing of the charged but not exposed zone b - c when the latter zone is beneath the brush 8, such a potential V.sub.2 may be applied to the brush that either there is no electric field between the magnetic brush and the photoconductor or there is only a weak electric field, the flux lines of which run in a direction opposite to those of the previously existing field, so that no toner passes to the photoconductor 2.

Switching from the normal developing voltage V.sub.1 to the voltage V.sub.2 takes place when the boundary line b between the exposed and unexposed zones a - b and b - c respectively has passed through the developing station. This change-over may be controlled through a switching means 9 by the impulse generator of the electrophotographic machine.

It has been found expedient to use a potential V.sub.2 that is somewhat higher than that of the charges on the zone of the photoconductor which is not exposed for producing an image. When this potential of the charges was -800 V, a potential V.sub.2 of -850 V prevented developing of a charged but unexposed zone of the photoconductor.

The process of the invention can also be used when a cascade system comprising a developing electrode is used instead of a magnetic brush developing unit. Apparatus suitable for use in this process is shown in FIG. 3. Potential V.sub.1 is applied to the developing electrode 10 when the zone a - b of the photoconductor is positioned below this electrode. On the other hand, potential V.sub.2 is applied to the developing electrode 10 when only photoconductor that is not exposed for producing an image is positioned beneath the electrode (zone b - c). The process of the invention and the apparatus for carrying it out offer the considerable advantage over previously proposed processes that only very low power is required for operating the developing electrode. Furthermore, the consumption of toner is considerably reduced without the need for an expensive and complicated quenching lamp. A further advantage is that the toner filter can remain in the machine for a longer period before having to be replaced.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

Claims

What is claimed is:

1. An electrophotographic process comprising:

electrostatically charging the surface of a photoconductor to form a uniform change, exposing a first zone of said charged photoconductor surface to a light image to form a latent charge image thereby leaving a second zone adjacent to said first zone uniformly charged,

passing said first and second zones past means for developing a latent charge image,

applying a first potential to said developing means to form a first electrical field between said developing means and said first zone while said first zone is passing so as to develop said latent charge image of said first zone and

applying a second potential, different from said first potential, to said developing means to form a second electrical field between said developing means and said second zone of said uniform charge of said second zone.

2. A process as in claim 1 wherein said step of passing includes passing said zones past a magnetic brush.

3. A process as in claim 1 wherein said step of passing includes passing said zones past a developing electrode of the cascade type.

4. A process as in claim 1 wherein said second potential includes applying a second potential has a magnitude less than the potential of the charge of the photoconductor in the second zone and said second electrical field is less than said first electrical field.

5. A process as claimed in claim 1 wherein said second potential includes applying a second potential has the same sign but greater than the potential of the charge of the photoconductor in the second zone and has the opposite sign to said first potential.

6. A process as claimed in claim 1 wherein said second potential has the same magnitude as the potential of the charge of the photoconductor in the second zone thereby reducing the electrical field to zero.