CONTAMINANT REMOVAL FROM A CORONA-BASED CHARGING DEVICE

BACKGROUND OF THE INVENTION

The present invention relates to contaminant removal from a corona-based

charging device, and, more particularly, but not exclusively to contaminant removal from a

scorotron, used in imaging, for example in xerographic devices such as laser printers or photocopiers or liquid electro-photography (LEP) devices.

A scorotron is a device for producing a coronal current for charging surfaces. The

scorotron comprises a charging wire, a conductive enclosing housing that encloses all but

one side of the scorotron, and a grid on the open side. In the scorotron the charging wire,

or corona emitting wire, is charged to very high voltage in order to form a corona. From

the corona, charged particles of appropriate polarity are accelerated to the surface requiring charging via the grid, which is itself charged. As well as providing acceleration of the particles towards the surface, the grid allows for control of the current and uniform

distribution of the particles on the surface. A current of charged particles continues until

equilibrium is reached. The scorotron is used in applications from medical imaging to

laser printing. In a laser printer the scorotron is placed over the photoconductive drum. The

scorotron wire is charged to -600 volts. A plasma forms around the wire and the grid is

charged to around -100V. Charged particles are accelerated towards the grid, from which

they continue onwards towards the photoconductive surface of the drum. In a static

system the particle flow would continue only until the voltage at the photoconductive

surface reaches equilibrium, however, the drum rotates in use so that equilibrium is never

actually reached with the photoconductive surface. The scorotron is placed prior to an

image writing head in terms of the direction of rotation of the drum, so that the freshly

charged surface is then presented to the writing head. The writing head scans the surface with one or more laser beams. The beams discharge the surface at points of contact to

produce a latent image, and then ink is applied to the surface at a unit called a binary ink developer (BID) unit, in accordance with the latent image. Toner particles are attracted to

the image areas and repelled from the non-image areas.

Now the inside of the scorotron is a highly reactive region in which high energy

particles form a plasma, the corona. Ozone and like reactive chemicals are formed in this reactive region and need to be removed, and removal is generally earned out using a

stream of air, directed at the corona, which removes particles for collection.

Now, the liquid electro-photography (LEP) press uses liquid ink which contains volatile substances. Furthermore, the scorotron is located in proximity to the ink developer unit, which means that fumes, such as hydrocarbon fumes of various kinds, from

the ink material find their way in appreciable quantities into the scorotron, where they

undergo chemical changes under the influence of the corona and form contaminants.

Materials from the ink are particularly volatile and ink is a major, if not the only, source of

contaminants.

During printing cycles the contaminants tend to gather on the walls and the grid of

the scorotron and when the machine is switched off they tend to leak, that is migrate, onto

the photoconductor. Once on the photoconductor the contaminants bring about local

changes in the properties of the photoconductor which may cause damage to any prints that

are being made, so that high quality printing can only resume after a number of cycles have

been gone through in order to dissipate the contaminants.

A number of methods are known for coping with the problem. One is simply to

operate the machine over several dummy print cycles until the contaminant dissipates from the drum. These dummy cycles may involve paper which is then thrown away, or

alternatively, null cycles may be used in which paper is not transported through the system.

The null cycle solution nevertheless wastes time and materials.

Another solution is to extract the scorotron directly after use and clean it using cleaning fluids or pressurized air. Again this takes time and is not preferred by customers.

A variation which takes less time is simply to extract the scorotron after work and then

reinsert it before commencing use, so that the scorotron is not present when the system is

idle and there is no chance for contamination of the drum to actually occur.

Yet another solution is to replace the photoconductor after long periods of rest.

None of these solutions is ideal, and there is thus a widely recognized need for, and it would be highly advantageous to have, a charging system devoid of the above

limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a wire corona

device for providing charging current to a charging surface, the device comprising: a charging wire for forming a corona; and

containing walls for containing said corona; and wherein decontamination media is applied to internal surfaces of said walls for

preventing accumulation on said walls of contaminants from said corona.

According to a second aspect of the present invention there is provided a

photoimaging device comprising a photoconductive surface and a charging device for

providing current of charged particles to the photoconductive surface, the charging device comprising:

a charging wire for forming a corona; and

containing walls for containing said corona; and

fluid direction structure for directing a stream of decontamination fluid to internal surfaces of said walls for preventing accumulation on said walls of contaminants from said corona.

According to a third aspect of the present invention there is provided a method of

operating a scorotron to prevent leakage of contaminant from the scorotron interior onto adjacent surfaces, the scorotron comprising a charging wire, enclosing walls and a grid, the method comprising:

applying high voltage to the charging wire, and

applying fluid flow to at least one of the group consisting of

1) at least parts of internal surfaces of said enclosing walls and 2) said grid.

According to a fourth aspect of the present invention there is provided a method of

constructing a scorotron to prevent leakage of contaminant from the scorotron interior onto

adjacent surfaces, the scorotron comprising a charging wire, enclosing walls and a grid, the method comprising:

providing a charging wire;

providing enclosing walls, said enclosing walls having internal surfaces;

providing a grid;

coating at least one of the group consisting of:

1) at least parts of internal surfaces of said enclosing walls and

2) said grid.

Unless otherwise defined, all technical and scientific terms used herein have the

same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples provided herein are illustrative

only and not intended to be limiting.

Implementation of the method and system of the present invention involves

performing or completing certain selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of preferred embodiments of the method and system of the present invention, several selected steps could be implemented by hardware or by software on any operating system of any

firmware or a combination thereof. For example, as hardware, selected steps of the

invention could be implemented as a chip or a circuit. As software, selected steps of the

invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In- any case, selected steps of the method

and system of the invention could be described as being performed by a data processor,

such as a computing platform for executing a plurality of instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is

stressed that the particulars shown are by way of example and for purposes of illustrative

discussion of the preferred embodiments of the present invention only, and are presented in order to provide what is believed to be the most useful and readily understood

description of the principles and conceptual aspects of the invention. In this regard, no

attempt is made to show structural details of the invention in more detail than is necessary

for a fundamental understanding of the invention, the description taken with the drawings

making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings:

Fig. 1 shows a charging, writing and reading system for a photoconductive drum

according to known prior art systems;

Fig. 2 shows a scorotron according to the known art;

Fig. 3 is an improved scorotron according to a first preferred embodiment of the

present invention.

Fig. 4 is a variation of the scorotron of Fig. 3 in which airflow is used as a

decontaminant, according to a preferred embodiment of present invention.

Fig. 5 is a variation of the scorotron of Fig. 3 in which a coating is used as a decontaminant, according to a preferred embodiment of the present invention.

Fig. 6 is a photograph of a scorotron half coated with a gold coating according to a preferred embodiment of the present invention and half left untreated.

Figs. 7 and 8 illustrate contamination experiments carried out with scorotrons that

were half coated and half left untreated.

DETAILED DESCRIPTION OFTHE PREFERRED EMBODIMENT The present embodiments comprise an apparatus and a method for preventing

contamination of the internal surfaces of the scorotron. Such contamination is one of the

causes of loss of image quality, particularly for initial images made following a period of

non-use.

The principles and operation of an apparatus and method according to the present

invention may be better understood with reference to the drawings and accompanying description.

Before explaining at least one embodiment of the invention in detail, it is to be

understood that the invention is not limited in its application to the details of construction

and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and

terminology employed herein is for the purpose of description and should not be regarded

as limiting.

Reference is now made to Fig. 1 which illustrates a prior art system for charging a photoconductive device, for use in a laser printer or photocopier. It will be appreciated

that the system is useful for any device that uses writing based on a laser beam but is more generally applicable to any device that requires a charged plate, irrespective of whether

there is writing involved.

A drum 10 carrying a photoconductive plate rotates about its axis. A scorotron 12

provides negative charge uniformly to the surface of the plate as it passes underneath. It

will be appreciated that in some systems positive charging may be deemed appropriate.

A write-head 14 uses one or more laser beams to discharge those parts of the

surface it falls upon, thereby to write image data onto the charged photoconductive

surfaces to form a latent image. Ink application unit 16 applies ink to the photoconductive

surface, the ink being directed by the latent image to form a real image on the drum. Ink

transfer unit 18 then transfers the image onto a blanket or directly onto the paper or other

printing medium. Discharging unit 20 discharges the photoconductive surface ready for

the next image.

Reference is now made to Fig. 2 which illustrates a cross-section of a scorotron device suitable for the system of Fig. 1. The scorotron 40 comprises a central corona wire

42 which is charged to high voltage and around which a corona forms. The scorotron is enclosed by a conductive enclosure wall 44 on all sides but one, and by a grid 46 on the

remaining side. The grid accelerates particles from the corona towards the

photoconductive surface. In practice the scorotron may be a two-chamber device with two

wires but a single chamber is here shown for simplicity. As explained in the background, the corona is a highly reactive region, and contaminants from the atmosphere, and particularly from the ink, may be converted into

substances that stick to the walls and grid. This is not in itself harmful but at times of non- use the contaminants may drip, leak or otherwise migrate onto the photoconductive plate.

The presence of the contaminants on the photoconductive plate leads to local variations in

the photoconductive properties of the plate and thus to a reduction in image quality. The

local variations may for example involve introduction of lateral conductivity or may change the local charging characteristics of the photoconductor.

Reference is now made to Fig. 3 which is a simplified diagram showing a first

preferred embodiment of a scorotron according to the present invention. The scorotron 50

of Fig. 3, is more generally a wire corona device for providing charging current. The scorotron includes a charging wire 52 which is provided with a voltage which is

sufficiently high that a corona or plasma is formed around the wire. The scorotron is a confined space defined by enclosing walls 54 as before and in accordance with the

preferred embodiment, decontamination media, as indicated by arrows 56, is applied to

internal surfaces of the walls and/or to the grid for preventing accumulation on said walls

of contaminants from said corona. The decontamination media may in one embodiment

be a stream of fluid, such as a stream of air, which is simply blown continuously or intermittently over the grid or the inner wall surfaces or both. The stream of air, or more

generally a stream of gas, blows away particles and thus prevents their accumulation over

the surfaces. It is noted that this stream of air is complementary to and not identical with

the stream of air that is in any event directed towards the corona at the center of the scorotron chamber.

In another embodiment the decontamination media may be a coating applied to the relevant surfaces. The coating may be an inert coating such as gold, or it may be a water

repellent or non-stick type of coating.

Reference is now made to Fig. 4 which is the same as Fig. 3 but which indicates directing structure 58, typically air pipes or air vents, for directing the stream of air

towards the internal surfaces. The use of air pipes is convenient as air flow in general is already available in the scorotron. As mentioned above, existing air flow is directed at the

central wire and the corona in order to remove ozone and the like from the area of the

corona. In some cases all that may be required is additional holes in the scorotron housing.

Reference is now made to Fig. 5, which shows an adaptation of the scorotron of

Fig. 3 to provide decontamination media in the form of a coating to the inner wall surfaces

and the grid surfaces. The coating is indicated by reference numeral 60. As explained, the

coating may be an inert coating such as gold, or a surfactant or like water repellant or non¬

stick coating. Materials that can be used include the surfactants Zion ™ and Zonyl ™ both

from Du Pont Corporation.

It is expected that during the life of this patent many relevant devices and systems will be developed and the scope of the terms herein, particularly of the term scorotron, is

intended to include all such new technologies a priori.

Reference is now made to Fig. 6, which illustrates a view from the underside of a

scorotron looking through the grid. The scorotron is half coated with gold (right hand side) and half left untreated.

Reference is now made to Fig. 7 which illustrates a comparative experiment using

a half-treated scorotron. The scorotron inner walls were half treated with Zion ™ material and half untreated. The grid was half treated with gold and half untreated. The lower part

of the figure shows left, center and right cross-sections of the scorotron and the upper part

of the figure shows contamination in a first image printed after a period of non-use. The left half of the grid is treated with the Zion ™ material and the right half of the grid is

untreated. The resulting marking is clearly more pronounced on the right hand side

corresponding to the untreated half. The same is shown in Fig. 8 for a scorotron in which the walls and grid were both

half coated in gold, in fact the scorotron shown in Fig. 6. Again the left hand part is treated and the right hand part is untreated. Again the resulting marking is clearly

asymmetric, with noticeably more marking on the right, untreated half.

It is appreciated that certain features of the invention, which are, for clarity,

described in the context of separate embodiments, may also be provided in combination in

a single embodiment. Conversely, various features of the invention, which are, for brevity,

described in the context of a single embodiment, may also be provided separately or in any

suitable subcombination.

Although the invention has been described in conjunction with specific

embodiments thereof, it is evident that many alternatives, modifications and

variations will be apparent to those skilled in the art. Accordingly, it is intended

to embrace all such alternatives, modifications and variations that fall within the

spirit and broad scope of the appended claims. All publications, patents, and

patent applications mentioned in this specification are herein incorporated in

their entirety by reference into the specification, to the same extent as if each

individual publication, patent or patent application was specifically and

individually indicated to be incorporated herein by reference. In addition,

citation or identification of any reference in this application shall not be

construed as an admission that such reference is available as prior art to the

present invention.