US3411482A

US3411482A - Electrographic toner development employing a clean-up electrode structure for removing unwanted background

Info

Publication number: US3411482A
Application number: US612531A
Authority: US
Inventors: Brodie Ivor
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-01-30
Filing date: 1967-01-30
Publication date: 1968-11-19
Anticipated expiration: 1985-11-19

Description

l. BRODIE 3,411,482 ELECTROGRAPHIC TONER DEVELOPMENT EMPLOYING A CLEAN-UP Nov. 19, 1968 ELECTRODE STRUCTURE FOR REMOVING UNWANTED BACKGROUND Filed Jan. 30. 1 67 FIG. I

PRIOR ART FIG. 2 PRIOR ART 4% fit L INVENTOR. IVOR BRODIE ATTORNEY United States Patent M ELECTROGRAPHIC TONER DEVELOPMENT EMPLOYING A CLEAN-UP ELECTRODE STRUCTURE FOR REMOVING UNWANTED BACKGROUND Ivor Brodie, Palo Alto, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 30, 1967, Ser. No. 612,531 4 Claims. (Cl. 118-637) ABSTRACT OF THE DISCLOSURE Electrographic toner developers or inkers are disclosed employing a relatively open clean-up electrode structure in the form of one or more slat-shaped electrodes or a metallic screen disposed over the charge image bearing surface of the recording medium being developed such medium being, for example, a strip of electrographic paper. A second electrode, in the form of a plate or the like, is disposed opposite the clean-up electrode structure on the other side of the recording medium. An electrical potential is applied to the clean-up electrodes to establish an electric field adjacent the image bearing side of the recording medium to pull charged toner particles from the inked surface of the recording medium which are not electrostatically bound to the charge image to be developed, thereby reducing the background.

Description of the prior art Heretofore, channel type toner developers or inkers have been employed to apply charged toner particles to charge images to be developed. Typical of such inkers is a channel member extending across the charge image bear-ing surface of an electrographic paper to be developed. The channel has a slot in one side adjacent the paper which permits the toner particles, suspended in a stream of insulative air or liquid, to come into fluid communication with the charge image to be developed. The charged toner particles are electrostatically attracted out of suspension and electrostatically bound to the charge image pattern thereby developing same. Increasing the concentration of toner particles in the suspension from a few percent by weight to higher concentrations permits faster development speeds. For example, for paper pulling speeds of 5 to inches per second employing a liquid toner, the toner particle concentration is preferably increased to about to by weight of the liquid toner. However, with such high concentrations of toner particles, it is found that substantial amounts of the toner particles are left on the paper in the regions where they are not electrostatically bound to the charge image to be developed. Therefore, they produce an unwanted background fog on the developed image.

Therefore, development electrodes, in the form of one or more plates disposed with their flat sides facing the image bearing surface being developed, have been employed to control the rate at which toner particles are deposited upon the charge image being developed. Such electrode structures are relatively closed in that the toner particles do not readily pass through the structure. Toner is supplied by a stream directed between the electrode structure and the image being developed. In a high speed inker or developer, such a closed electrode structure interferes with an adequate supply of the toner particles to the image, thereby leaving the image underdeveloped. Such development electrode structures are described in a text entitled, Electrophotography, published in 1965 by The Focal Press of London and New York at pages 32-34 and 152.

3,411,482 Patented Nov. 19, 1968 Summary of the present invention The principal object of the present invention is the provision of means for reducing or eliminating background fog on toner developed electrostatic images.

One feature of the present invention is the provision in an electrographic toner developer or inker of a relatively open clean-up electrode structure for producing an electric field over the image bearing surface of the recording medium for pulling charged toner particles off the background image areas where they are not electrostatically bound to the charge image being developed, whereby unwanted background fog is removed from the developed image.

Another feature of the present invention is the same as the preceding feature wherein the clean-up electrode structure includes an elongated slat-shaped electrode directed across the recording medium with its thin edge facing the image bearing surface of the recording medium, whereby an intense clean-up electric field is produced adjacent the image bearing surface.

Another feature of the present invention is the same as any one or more of the preceding features wherein the inker includes a hollow channel member directed across the image bearing surface to be developed and such channel including an apertured side adjacent the image to be developed for passage of the charged toner particles to the image and wherein at least a portion of the clean-up electrode structure is disposed at the trailing marginal edge of the apertured channel for cleaning up the background as the recording medium is caused to traverse the channel.

Another feature of the present invention is the same as any one or more of the preceding features wherein the clean-up electrode structure includes a grid disposed across the mouth of the apertured side of the hollow inking channel.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a schematic perspective view of an electrographic device employing the prior art developer,

FIG. 2 is an enlarged sectional view of the structure of FIG. 1 taken along line 22 in the direction of the arrow,

FIG. 3 is a view similar to that of FIG. 2 depicting the clean-up electrode structure of the present invention,

FIG. 4 is an enlarged detail view of a portion of the structure of FIG. 3 delineated by line 44, and

FIG. 5 is a view similar to that of FIG. 3 depicting an alternative embodiment of the present invention.

Description of the preferred embodiments Referring now to FIG. 1, there is shown one example of a prior art electrographic device 1. In this example, an elongated hollow inking channel or developer 2 is employed for applying liquid toner to charge images 3 deposited upon the charge retentive surface 4 of electrographic recording paper 5. The charge images 3 are deposited by a moving electric stylus 6 carried by a conductive cable 7 and moved back and forth across the recording paper 5 by means of a conductive drive pulley 8 and an idler pulley 9. The drive pulley 8 is driven from the output shaft of the reversible rebalance motor of a self-balancing potentiometric recorder circuit, not shown. The transverse position of the stylus 6 on the recording paper 5 is determined by the amplitude of the signal to be recorded by the recorder 1.

A negative writing potential as of 500 v. is applied to the stylus 6 from a source 11 via the conductive pulley 8 and cable 7. A grounded plate-shaped auxiliary writing electrode 12 is disposed beneath the stylus 6 on the opposite side of the recording paper 5. The potential difference across the gap between the end of the stylus 6 and the plate-shaped auxiliary electrode 12 is sufficient to produce a line-shaped charge image pattern upon the charge retentive coating 4 of the recording paper 5. The recording paper actually includes a conductive paper backing layer with the dielectric charge retentive layer formed thereon. In this manner, the very thin, i.e., 4-micron thick, charge retentive layer can be adequately supported.

The recording paper 5 is pulled past the inking channel 2 by means of a pair of friction drive wheels comprising an idler wheel 13 and motor driven drive wheel 14. The inking channel 2 comprises a hollow channel member 15, as of Lucite plastic. The bottom side wall of the channel 15 is provided with a longitudinally directed slot 16, as of 0.250 wide, extending nearly the entire length of the channel 2 and nearly across the width of the recording paper 5, as of 4" to 11" wide. An input pipe 17 and an output pipe feed a stream of electrographic toner through the inking channel 2 from a reservoir of toner, not shown.

The electrographic toner comprises a colloidal suspension of charged toner particles in an insulative liquid vehicle. Although the toner particles may have either a positive or a negative charge they are positively charged in preferred embodiment.

Also, in a preferred embodiment, the toner liquid in the inking channel 2 is at less than atmospheric pressure such that the paper 5 is pushed by atmospheric pressure up against the marginal edges of inking slot 16 sealing same against escape of liquid toner and assuring that the charge image is brought into intimate contact with the toner liquid within the channel 2, see FIG. 2. As the paper 5 is drawn past the inking slot 16, the positively charged toner particles are attracted out of suspension and electrostatically bound to the negative charge image 3 to be developed on the charge retentive surface 4 of the recording paper 5. When relatively high concentrations of toner particles are used for high speed inking or development as, for example, 15% to 30% by weight of the liquid toner, it is found that some of the positive toner particles are left on the non-charge bearing surfaces, i.e., background areas of the image to be developed. These particles produce an unwanted background fog.

Referring now to FIG. 3, there is shown a clean-up electrode structure 21 for removing the unwanted background fog. The clean-up electrode structure 21 comprises an array of parallel directed slat-shaped electrodes 22 which extend lengthwise of and across the inking slot 16 from one end to the other. The slat-shaped electrodes 22 are electrically connected together via lead 23 which is connected to a source of negative voltage 24, as of 250 v., via a switch 25. A segment of the auxiliary plate-shaped electrode 12' is spring biased, as indicated by the arrow 26, to force the plate into electrical contact with the conductive backing of the recording paper 5. Under these conditions, a series of intense electric fields are produced over the image bearing surface of the recording paper 5 within the inking slot 16, as more clearly shown in the detail of FIG. 4. These regions of intense electric field cause the positively charged toner particles which are not electrostatically bound to the charge image to be pulled from the background areas of the image and attracted to the clean-up electrodes 22. The clean-up electrode structure 22 is relatively open. As used herein relatively open means that the electrode structure 22 has a projected area on the image bearing surface being developed of less than 50% of the development area of the slot 16, i.e., the area of the image hearing surface exposed to the toner. The thin edge portions 27 of the clean-up electrodes 22 are preferably slightly recessed, as by for example, 0.002, from the marginal edges 28 of the slot 16 to prevent the accumulation of charged toner particles. on the electrodes 22 from being carried away by the paper 5 as it is pulled past the electrodes 22.

In a typical example, the slat-shaped clean-up electrodes 22 are 0.010 thick. The spacing between adjacent ones of the electrodes 22 should be sufiiciently large as of, for example, 0.15" to permit an adequate flow of toner to the image bearing surface 4 of the recording paper 5. As can be readily appreciated, the trailing edge electrode 22' is the most eflicient in that with the other electrodes additional background can be deposited after they have performed their cleaning function. Accordingly, adequate performance is obtainable by use of only the trailing clean-up electrode 22'. A particularly advantageous electrode arrangement employs only a leading edge electrode 22" and the trailing edge electrode 22'.

In operation, the clean-up potential is preferably applied only during development of charge images in order to prevent building up excessive accumulations of charged toner particles on the clean-up electrodes 22. Accordingly, the switch 25 may be switched to a grounded position 31 during non-development times to prevent such accumulations and to permit previously accumulated toner particles to be dispersed from the electrodes 22. Alternatively, the potential applied to the second position of the :switch 25 may be positive to help drive off the accumulated particles. In a continuously operating developer 2, the switch 25 may be switched to position 31 periodically. The switching cycle may be determined by a timer, not shown, or by a device such as a cam carried on or geared to the paper pulling

friction drive wheels

13 and 14.

Referring now to FIG. 5, there is shown an alternative clean-up electrode structure 32. In this case, the apparatus is essentially identical to that of FIG. 4 except that the clean-up electrodes 22 are replaced by a grid of conductive wires 33 parallel to the longitudinal axis of the channel 2. A few support wires may be employed traversing the slot 16 which wires are preferably not parallel to the direction of motion of the recording paper 5. The grid 33 is relatively open and the unwanted background toner particles are pulled from the paper 5 to the grid which is afiixed over the mouth of the inking slot 16.

Although the inker 2 with the clean-up electrode structure of the present invention has been described as employed in conjunction with an electrographic strip chart recorder 1, it may be employed to advantage in any electrogrophic device, such as for example, a microfilm printer, oscilloscope camera or the like, where it is desired to prevent unwanted background fog on the developed images.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an electrographic apparatus, means for depositing an electric charge image to be developed on a charge retentive surface of a recording medium, a hollow elongated development channel member directed across the image bearing surface of the recording medium and having an apertured side adjacent the image bearing surface of the recording medium for applying a suspension of charged toner particles in a fluid medium to the charge image on the recording medium thereby developing same, said channel member defining a development area wherein the charge image is contacted by the fluid toner, said apertured side of said channel forming a fluid passageway through which the toner particles flow from said channel to the charge image to be developed, means for supplying the fluid toner to the hollow interior of said channel member, a clean-up electrode structure having portions disposed immediately adjacent the development area of the image bearing surface of the recording medium, said immediately adjacent clean-up electrode structure having a projected area on the development area of less than 50% of the; development area, means for applying an electrical potential to said clean-up electrode structure of such a magnitude to produce an electric field for pulling the chargedgtoner particles from the background'isurface of the recording medium Where the toner particles 'are not electrostatically bound to the charge image to be developed, said clean-up electrode structure includingaa slatshaped electrode turned on its edge such that the thin marginal edge portion of the electrode faces the charge image bearing surface of the recording medium to produce a relatively intense clean-up electric field. the improvement comprising, means for causing said recording mediu'rn to pass said apertured side wall of said channel member to define leading and trailing marginal side edges of said apertured side wall of said channel member, said side edges of said channel being spaced apart in the direction of movement of said recording medium past said channel with said trailing edge being spaced from a leading edge in the direction of movement of said medium, and said clean-up electrode structure including a slat-shaped clean-up electrode disposed at said trailing marginal edge of said channel member. v

2. The apparatus of claim 1 wherein said liquid toner is supplied to said hollow channel at less than atmospheric pressure, and wherein the recording medium is a web which is pushed by atmospheric pressure against the marginal edges of the apertured side wall of said hollow channel thereby sealing same against escape of liquid toner.

3. The apparatus of claim 1 wherein said clean-up electrode structure includes a plurality of elongated electrically conductive slat-shaped members disposed across the fluid passageway in the apertured side wall of said channel, said elongated .conductive membersj'being directed in the direction of the longitudinal axis of said channel member and across said recording web.

4. The apparatus of claim 1 wherein said. recording medium includes a conductive backing portion and a dielectric charge retentive surface, and wherein said means for supplying the liquidsuspension of toner supplies a positively charged colloidal suspension of such toner particles, and wherein said means for applying an electrical potential to said clean-up electrode structure supplies a potential to a portion of said electrode structure disposed on the developed-side of said charge image which is negative relative to the potential applied to the conductive backing portion of said recording medium.

References Cited UNITED STATES PATENTS 2,956,494 10/1960 Tyler et al 118-637 3,147,147 9/1964 Carlson 118-637 3,154,546 11/1964 Dirks 118-637 3,203,395 8/1965 Liller 118-637 3,299,787 1/ 1967 Kolb et al.

3,342,164 9/1967 Lewis 118-637 3,345,925 10/ 1967 O stensen 118-637 XR PETER FELDMAN, Primary Examiner.