US3058444A - Apparatus for developing electrostatic image on electrographic sheet - Google Patents

Description

Oct. 16, 1962 M. SUGARMAN, JR.. ETAL APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE ON ELECTROGRAPHIC SHEET 2 Sheets-Sheet Filed March 13, 1959 film elecroa'e W///////////////////// g mafne meg er LSW S Mam/177 Le Vzne BY J 05 2, #MM/ Ve# 9- OAJM/ Oct. 16, 1962 M. L. SUGARMAN, JR.. ETAL 3,

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE ON ELECTROGRAPHIC SHEET Filed March s, 1959 2 Sheets-Sheet 2 vo id 9 CW A-r-rvs.

United States Patent Ofifice 3,058,444 Patented Oct. 16, 1962 3,53,444 APPARATUS ELGPING ELECTRQSTATEC ON SHEET Meyer L. Sugarman, Jr., Wilmette, and Marvin B. Levine,

Evaston, 111., assignors to American Photocopy Eqni ment Company, Evanston, ill., a Corporation of Illinois Filed Mar. 13, 1959, Ser. No. 7%,3-%5 2 claim& (Ci. 118-437) The present invention relates to electrophotographic printing and more particularly to means for insuring a clean background in the development of latent electrostatic images.

The electrophotographic printing process employed in ofiice photocopy machines and the like involves the steps of uniformly charging an electrographic sheet, exposing the sheet to produce a latent image thereon with dissipation of charge from the light-struck areas, and development of the image by applying a pigmented powder or toner which tends to adhere selectively to the charged areas. Following this the toner is either fixed in place on the sheet or transferred to a second sheet. One difficulty which has been experienced with this procedure is that the charge is not wholly dissipated from the light-struck areas, usually the background areas, with the result that the background areas have toner spuriously deposited thereon to a degree which makes such background areas grey and muddied rather than white and crispf To reduce this unwanted deposit of toner in the background areas, it has been proposed to contact the back of the sheet with an extensive auxiliary electrode and to apply a potential between the electrode and the reservoir of toner particles which is sufiicient to neutralize the effect of the residual charge. Practical experience with such an arrangement has shown that the results are often unsatisfactory, with only partial removal of the toner particles, with unwanted variations in density in both the background and printed areas, and with the occurrence of dark stripes across the surface of the developed copy. These effects tend to vary with conditions of humidity and temperature, with the amount of "ripple in the sheet, and with the mechanical adjustment of the copying machine, particularly the paper tension adjustment in the case of web-fed machines. The final result is unpredictable in the day to day operation of a given machine, or even from sheet to sheet in a sheet fed machine. Moreover, the auxiliary electrode gives rise to complications where it is desired to copy sheets of less than standard width, i.e., less than the width of the electrode. The above has tended to discourage use of auxiliary charged electrodes for clean-up purposes in machines intended for ofi ice and commercial use.

Accordingly, it is an object of the present invention to provide an arrangement for developing an electrographic sheet which insures uniform clean-up of background or "white" areas and which is free of the disadvantages normally associated with the use of auxiliary charged electrodes. It is a more specific object to provide a developing arrangement employing an auxiliary charged electrode but which gives uniform clean-up under all ambient conditions and regardless of the condition of the paper. That is to say, uniform results are achieved over wide variations in the resistivity of the paper and independently of the amount of ripple, pucker or other surface irregularities, within the practical Variations encountered in normal practice. It is an important object of the invention to provide a developing arrangement producing uniform clean-up in sheet-fed machines where problems of ripple have been aggravated because of inability to tension the paper during development.

It is another object to avoid the stripes and smears over both the background and copy which have been characteristic of conventional electrographic copy machines.

It is a further object of the invention to provide a developng arrangement employing an auxiliary charged electrode but which is capable of accepting sheets of less than standard width, which are substantially narrower than the width of the developer brush and electrode without affecting the operation or the quality of the result.

It is a final object of the invention to provide a worthwhile improvement in the developing procedure characterized above, which is universally applicable to various developing setups employing photoconductive sheets, whether manual or automatic, and which may be incorporated in new machines and existing machines at extremely low cost.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

PIGURE 1 shows in diagrarnmatic perspective an embodiment of the invention as applied to either a sheet-fed or a web-fed copying machine.

FIG. 2 is a fragmentary section taken along the line 2-2 of FIG. 1.

FIG. 3 is a diagram showing charging of the sheet by electrostatic induction.

FIG. 4 is a dagram similar to FIG. 3 but absent the insulating film.

FlG. 5 is a diagrammatic perspective of a modified developing arrangement employing the present invention.

FIG. 6 is a diagramrnatic perspective showing the invention employed in connection with a hand developing brush.

While the invention has been described herein in connection with several preferred embodiments, it will be understood that we do not intend to limit the invention to the illustrated embodiments but intend to cover such alternative and equivalent arrangements as may be included within the spirit and scope of the appended claims.

Referring now to FlG. 1, a development arrangement 10 is shown which serves to develop a latent image on an electrographic sheet 12. This sheet, in the present embodiment, includes a paper backing portion 13 and a photoconducting portion 14 in the form of a thin resin-bound layer. The photoconductor may, for example, consist of zine oxide or selenium, dispersed in a resin binder to create a fiexible photoconductive surface. Both of the latter materials have the property of acqiring a uniform electrostatic surface charge when subjected to a high potential field and are capable of dissipating such charge selectively in the light-struck areas when exposed to a photographic image. It will be assumed that the electrographic sheet 12 has been previously charged and exposed to a photographic image so that upon entering the development apparatus the paper includes a latent electrostatic image exhibiting maximum charge in those areas which have not been struck by light.

In carrying out the development, the electrographic sheet is drawn by rollers (not shown) upwardly adjacent a developer brush 20. The brush arrangement may, for example, be of the Construction shown in copending application Serial No. 757,779 filed August 28, 1958, and assigned to a common assignee. Briefiy stated, the brush includes a magnetic roller 21 having provision for rotating the same during operation of the machine and having adherent bristles 22 formed of a loose, dry mixture of magnetic powder and toner. The roller 21 rotates in a trough 23 which serves as a reservoir for the developing material. A flexible felt strap or other suitable seal 23a may be utilized to allow paper to pass into the developing unit while retaining all of the developer mix therein.

As is well known to those skilled in this art, the toner particles tend to adhere triboelectrically to the magnetic particles which act as a carrier but leave the magnetic particles, due to electrostatic attraction, when brushed against the charged areas of the sheet. In order to insure intimate contact between the brush and the sheet, a closed magnetc circuit is employed, with the return path on the opposite side of the sheet in the form of a bar 24. The ends of the bar 24 are magnetically .coupled to the ends of the magnetic roller 16 through magnetc end pieces 25 so that the roller is of one polarity and the bar of the other, with magnetic lines of force extending between the two.

In operation, as the sheet 12 leaves the developer brush, the toner creates an image thereon consistng of printed areas, for example, as indicated at 26, surrounded by white or background areas 27 which are, or should be, free of toner. In accordance with the conventional practice, the toner or the surface of the electrographic sheet, or both, may include a thermoplastic binder, with the sheet being subjccted to radant heat as a final step for the purpose of fixing the particles of toner in place, thus producng a permanent copy.

In accordance with the present invention, an extensive auxilary electrode is employed which is arranged parallel to the back of the sheet, and a dielectric layer, preferably in the form of an insulating film, is interposed between the electrode and the sheet so that, when a high direct potential is applied to the electrode, the sheet is charged, not by contact with the electrode, but purely by electrostatic induction. This charge is applied so as to neutralize undesirable charge in background areas, which may be left after exposure, or may be created by friction of the magnetic brush itself against the coated surface.

In the present embodiment the auxiliary electrode is in the form of a flat conductive plate 30 which preferably extends beyond the lateral edges of the sheet 12. The dielectric layer indicated at 31 is preferably in the form of a film 'of plastic or other insulating material having a high specific resistance and a relatively high dielectric constant. The thickness ther'eof may be from a fraction of a mil to several mil s, although thicknesses on the order of of an inch or even greater have been used without affecting successful operation. The nature of the dielectric material is not critical and polystyrene, cellulose acetate and polyester resins, an example of the latter being marketed under the name of Mylar, have been employed with good results. All of these materials have relatively high dielectric constants.

As to the charging circuit, a high voltage source indicated at 35 is used which may be of any desired type capable of 'producing voltages on the order of several hundred to several thousand volts, but with low current capability and relatively high internal resistance. Assuming that the electrostatic image consists of negative charges, the voltage applied' to the auxiliary electrode is of positive polarity. The opposite terminal'is'formed by the developer brush and toner thereon, the brush assemhly and voltage supply having a common ground connection.

According to one of the aspects 'of the invention, the thickness and dielectric constant of the film 31 is such that the potential drop through the film greatly exceeds the potential drop through the sheet 12 and a potential is used between the electrodes which is many times greater than that which is conventionally employed, absent the insulating film. For example, in a conventional arrangement the potential between the'electrode and brush may be on the order of 200'volts, whereas in the present device a potential in excess of 1000 volts is used, preferably' a potential on the order of 4000 volts. 'Voltages of this order may be easily obtained using modern techniques, even in machines small enough for ofi'ice' desktop use, because of the low current drain; moreover, such voltages are safe since the electrode 30 is substantially completely enclosed by the insulating dielectric film 31.

In analyzing the device for the purpose of explaining the features and advantages, it is desirable to consider the two members 20, 30 as the electrodes of a capacitor having a plurality of dielectric layers. Referring to the diagrammatic FIG. 3, the first dielectric layer is the film 31 and the second dielectric layer is the sheet 12, the film 31 preferably having higher dielectric constant than the material, usually paper, which comprises the body of the electrographic sheet, and hence a dielectric constant nany times that of air. Thus, employing a dielectric film having the same order of thickness as the sheet itself, most of the potential drop between the plates of the capacitor will occur in the dielectric film. For example, where a voltage source is used having a terminal voltage of 4000 volts, a drop of 3800 volts may occur in the dielectric film with the balance of the drop, on the order of 200 volts, occuring in the electrographic sheet.

It is to be noted in connection with FIG. 3, that the electrographic sheet 12 is kept out of contact with the charging electrode 30 and is charged simply by electrostatic induction. This mode of charging has been found to result in a uniform charge being applied to the sheet 12, as diagrammatically indicated, quite independently of rippling or puckering of the sheet, producng air pockets such as indicated at 38, and also independently of gross or localized variations in the'resistivity of the sheet which may take place under various ambient conditions. Since a uniform charge is thus applied to the sheet, the neutralizing eifect will be uniform over the entire area of the sheet. To insure clean-up of the background areas, the voltage of the supply is simply set at a value sufficient to dispose of the residual negative charge existing in the white or background areas. The voltage adjustment may be made at the factory for the conditions of use andthereafter need not be changed.

It is to be particularly noted' that substantially uniform V charging occurs in spite of the existence of air pockets such as that shown at 38. Analysis indicates that the reasons for this is that the air, having a relatively low dielectric constant, produces only a minor potential drop as compared to the potential existing across the dielectric film so that even a relatively large air pocket produces only'a minor percentage change in the potential assumed by the sheet in the affected area. For purposes of comparison, reference is made to the diagram of FIG. 4 which illustrates a prior art arrangement in which the electrographic sheet indicated at 12a is in direct contact with'the charged electrode indicated at 30a. The electrode is capable of applying to the'sheet a positive charge in those areas of the sheet where direct contact exists as for example at 12b and 126. However, in the presence of a ripple creating avoid or air pocket 38a between the electrode and the sheet, and assuming that ambient conditions'are such that thesheet is poorly conductive, there Will be an absence of charge, and hence of neutralizng eifect, in the aifected'areas of the sheet giving rise to unwanted variations in clean-up and even to stripes and smears across'the background and printed areas.

While the present construction produces uniform neutralization -o-f residual charge even in the presence of air pockets resulting from rippling or puckering of the sheet, it is nevertheless desirable to minimize the air pockets in a web-fed machine by causing the sheet to ride in supporting contact directly on the dielectric film, and the rollers (not shown) which carry the sheet may be readily positioned to draw the sheet rather tightly against the film.

However, in the case of a sheet-fed machine the present invention is particularly useful since the sheets are largely self-supporting and cannot be flattened by tensioning between rollers or the like. r

. It is 'one of the features of the present invention that sheets having different widths may be accommodated, even though the width is less than the width of the charging electrode'and brush'assembly. Thus, beyond the ed ges of the sheet the brush idly contacts the surface of the dielectric film but, because of the insulation provided by the film, no current fiows. This 'is to be contrasted with prior' devices in which the portions of the brush lying outside the boundary of the sheet contact the electrode and thus tend to short circuit the voltage supply, resulting in a substantial drop of Voltage so that the effect of the auxiliary electrode is lost. Moreover, it is to be noted that the insulation constantly provided by the dielectric film makes it impossible to load the voltage supply 35 so that a light-duty, inexpensive and compact voltage source may be used with perfect reliability. Also, there is no need to divided the electrode into narroW sections with such sections being supplied through independent dropping resistors in an etfort to avoid the above mentioned short circuiting effect as has been proposed. ln sum, we have devised a developing arrangement which overcomes all of the disadvantages of prior practices but which is nevertheless of inherently low cost.

While the invention has been described in connection With a mechanized, continuous Web-fed machine, it Will be apparent to one skilled in the art that our teachings are not limited to the construction shown in FIG. 1, but are equally applicable to other developing arrangements as, for example, diagrammatically shown in FIGS. and 6. The arrangement of FIG. 5 difiers from that of FIG. 1 primarily in the Construction of the developer brush and in the respect that the sheet being acted upon is separate rather than in the form of a continuous web. The developer brush Construction indicated at 40 includes a first magnetic bar 41 and a second magnetic bar 42 arranged on opposite sides of the sheet and magnetized by permanent m'agnets 43, 44 interconnecting the ends of the bars. A mixture of toner and magnetic particles forms the bristle portion 45 of the brush. In contact with the developer brush is an electrographic sheet 50 which corresponds to the electrographic sheet 12 in the previous embodiment. This sheet is supported on a dielectric film 51 covering the charging electrode 52, the latter being charged by a high voltage supply 53. In use, means may be provided for moving the sheet on its support relative to the developer brush or for moving the brush, so that the brush is drawn progressively along the sheet 50 to develop the :latent image on the letter, following which the deposited toner is fixed by heat just described in the preceding embodiment.

During the development the electrostatic charge induced in the sheet by the electrode 52 serves, as described, to insure clean-up of the background areas.

An even Simpler arrangement for employing the present invention in one of its aspects is illustrated in FIG. 6. Here a manual brush 60 is used in the form of a bar magnet having bristles 61 which contact the surface of the electrographic sheet 62. The latter is supported on a dielectric film 63 which covers the charging electrode 64, with voltage being applied to the oharging electrode by a voltage supply 65. In operation, a sheet 62 having a -latent electrostatic image is placed upon the film 63 following which the developer brush 60 is manually passed over the sheet. Electrostatic charging of the sheet occurs simultaneously with development until all of the area has been developed, following which the image is fixed in the usual way. Because of the insulating effect of the film 63, care need not be eXercised to keep the brush within the bounds of the sheet 62.

We claim as our invention:

1. In an arrangement for developing an electrographic sheet having a previously applied latent electrostatic image on the front surface thereof, said electrostatic image including changed and dischargecl areas, the latter having residnal charges thereon, the combination comprising a developer brush made up of nagnetic particles and toner particles, means for magnetizing the brush so that it extends into brushing engagement with said sheet, means for relatively moving the sheet so that toner particles are deposited thereon `in the electrostatically charged areas, an extensive auxiliary electrode arranged parallel to the back of the sheet, dielectric means interposed between the electrode and the back of the sheet, said dielectric means being coextensive with the entire surface of the electrode thereby assuring insulation of the electrode from the toner applying brush, means for applying a direct potential between the developer brush and the electrode, the potential being sufficiently high so that a potential gradient is set up in said sheet by electrostatic induction for removing residua-l charges from the discharged area of the electrographic sheet.

2. In an arrangement for developing an electrographic sheet having -a previously applied latent electrost atic image on the front surface thereof, the combination comprising a developer brush made up of magnetic particles and toner particles, means for magnetizing the brush so that it extends into brushing engagement with said sheet, an extensive auxiliary electrode arranged parallel to the back of the sheet, a dielectric film interposed between the electrode and the back of the sheet, said dielectric film being coextensive with the entire surface of the electrode thereby assurinig insulation of the electrode from the toner applying brush, means for relatively moving the sheet so that toner particles are deposited thereon in the electrostatically charged areas and With the sheet in contact with the film, means for applying a direct potential between the developer bnsh and the electrode with the brush maintained at ground potential so that a potential gradient is set up in said sheet by electrostatic induction and maintained independently of electrical contact between the sheet and the electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,857,290 Bolton Oct. 21, 1958 2,889,758 Bolton June 9, 1959 2,890,968 Giairno June 16, 1959

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