US3672298A - Ink mist suppression for a rotary printing press - Google Patents

Abstract

An ink mist suppressing arrangement for a printing press having a resiliently covered roller and cooperating cylinder carrying a film of ink and rotated at high speed in which the roller is insulated from the cylinder and charged with high voltage for the setting up of an electric field between the roller and cylinder for acting upon and returning ink mist particles to the film. The resilient surface layer on the roller may be formed of conventional material, for example synthetic rubber and in conventional thickness. No auxiliary wire or similar electrode, such as commonly used for establishing a corona discharge, need be employed.

Description

[ 51 June 27, 1972 [54] INK MIST SUPPRESSION FOR A ROTARY PRINTING PRESS [72] Inventor: Robert B. Reif, Grove City, Ohio [73] Assignee: North American Rockwell Corporation, El

Segundo, Calif.

22 Filed: Jln.29, 1970 211 Appl.No.: 6,677

[52] U.S.Cl ..101/350 [51] lnt.Cl. .341! 31/00 [58] Field of Search ..101/349, 350, 206, 207, 148, 101/147, 216, 219, DIG. 13, 208-210, 351-363, 1 416; 118/639 [56] References Cited UNITED STATES PATENTS 3,011,435 12/1961 Jones et al. ..101/350 2,558,900 7/1951 Hooper ..l0l/2l9 3,316,879 5/1967 Washburn.. 101/349 UX 2,558,901 7/1951 Hooper ..l01/219 HIGH VOLTAGE /7 8O TO OTHER INK ROLLERS 2,151,390 5/1939 Peterson 101/152 3,477,369 11/1969 Adamson et al ..l01/153 FOREIGN PATENTS OR APPLlCATlONS 1,159,923 7/1969 Great Britain 101/1 70 Primary Examiner-J. Reed Fisher Attorney-John R. Bronaugh [57] ABSTRACT An ink mist suppressing arrangement for a printing press having a resiliently covered roller and cooperating cylinder carrying a film of ink and rotated at high speed in which the roller is insulated from the cylinder and charged with high voltage for the setting up of an electric field between the roller and cylinder for acting upon and returning ink mist particles to the film. The resilient surface layer on the roller may be formed of conventional material, for example synthetic rubber and in conventional thickness. No auxiliary wire or similar electrode, such as commonly used for establishing a corona discharge, need be employed.

2 Claims, 6 Drawing Figures PATENTEnJum I972 3. 672.298

INVENTOR Rosear 5. Ram

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PMENTEDJUNZ'! 1972 SHEET 2 BF 2 T Irwsuroa RQBERT 5. Ram

A'r'rvs INK MIST SUPPRESSION FOR A ROTARY PRINTING PRESS The mistingof ink in a printing pressroom has been recognized as a problem for many years. Each printing press has numerous rollers and cylinders operating at high speed for transmitting, and evenly distributing, an ink film. Separation of the film at the exit nips apparently causes strands of viscous ink to be formed, which strands are quickly drawn to the point of breaking to release particles of ink into the air so small as to become airborne and which settle on'all exposed surfaces in the press room and within the press, creating a problem of cleanliness and safety most difficult to combat. Efforts have been made to alleviate the .problem including constant evacuation of the air within a press and use of electrodes of various configuration either to attract the ink particles or to charge the ink particles in an electric field so that they tend to be redeposited on the surface of the rollers and cylinders in the press. Prior charging electrodes have, for example, taken the form of a wire stretched in the vicinity of an exit nip which is energized to provide both a corona discharge for charging the ink particles and, to set up an electric field for acting upon such particles.

It is an object of the'invention to provide an ink mist suppressing arrangement for a printing press which is efi'ective in correcting a mistingc'ondition at its source so that mist is not deposited in the press in the form of an unwanted film or on surfaces within the pressroom. It is a related object to provide a mist suppression arrangement which causes the particles to be redepositedon the film on the surface of the rollers and cylinders promptly after. formation so that there is no need to remove or filter the air which carries the particles.

It is the main object of the present invention to provide an ink mist suppression arrangement employing high voltage for the control of the mist particles but which does not require use of any special electrodes such as wires or the like, either for creating the field or for charging the particles to be acted upon by the field. Indeed, it is an object to provide a suppression arrangement which avoids use of any special charging electrodes, which does not rely upon corona discharge, and which, instead, makes use of the small charge casually acquired by each of the particles as it is formed. As a result of the avoidance of special electrodes, there is no interference when cleaning the rollers or cylinders, there are no wires to rig or maintain; shock hazard to the press operator or maintenance personnel is minimized. Then, too, since corona discharge and leakage currents may be reduced to an extremely low value, depending in part upon the voltage and other conditions, power consumption is reduced to such a low level that the cost per hour of operation is inconsequential.

It is a still further object of the present invention to provide an ink mist suppression arrangement which is more efficient than prior devices, which is effective at all of the roller nips, regardless of direction of rotation, even in presses where the rollers occupy a folded configuration and where a portion of the nips have heretofore been inaccessible to wires or other electrodes. It is a related object to provide a suppression arrangement which may be applied to existing designs of presses as well as to new presses almost universally and at extremely low cost and which permits continues usage of existing roller surfacing materials in conventional thickness.

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

FIG. 1 is a diagram in the nature of an end elevation of a typical printing press to which the present invention is applicable, with the end frame removed to show the locations of the rollers and cylinders which comprise the inking path from the fountain to the plate cylinder;

FIG. 2 is a fragmentary end view of a portion of the ink path showing an ink cylinder and two cooperating inking rollers with attached high voltage supply;

FIG. 2a is a fragment showing the profile of the exit nip and the formation of individual ink particles;

FIG. 2b is a diagram similar to FIG. 20 but showing the general nature of the field which is established between the roller and cylinder;

FIG. 3 shows a typical ink roller assembly with means for insulating the stub shafts; and

FIG. 4-shows one end of the assembly of FIG. 3 mounted in a suitable roller socket.

While the invention has been discussed in connection with a preferred embodiment, it will be understood that I do not intend to limit the invention to the particular embodiment set forth but intend on the contrary to cover the various alternative and equivalent constructions properly included within the spirit and scope of the appended claims.

Turning now to FIG. 1 there is shown a press 10 having a first printing couple consisting of a printing cylinder 11 and a cooperating impression cylinder 12 acting upon a continuous paper web 13. Inkfed from an ink fountain 20 is formed into a film and passed upwardly through a series of rollers and cylinders for final application to the surface of the printing cylinder 11, the rollers and cylinders in the ink feed system being indicated at 21-27 respectively. The opposite side of the press includes a second printing couple and associated inking mechanism, corresponding elements being indicated by the same reference numerals with the addition of subscript a. In operation ink is forced under pressure from the ink fountain 20 onto the first, or fountain cylinder 21 and thence via a roller 22, cylinder 23, and roller 24, to a final cylinder 25 from which the ink film is passed to the plate cylinder via form rollers 26, 27. All of the rollers and cylinders forming the ink transfer path are mutually wetted" with ink. A portion of the rollers may be vibrated, but the means for securing the endwise movement is outside of the scope of the present invention.

Referring to FIG. 3, which shows a typical ink roller 26, the roller has a body 30 of cylindrical shape, made of steel, having alined stub shafts 31, 32 at the ends and an outer surface 33. The stub shafts are mounted in bearings 35, 36, respectively, which may, for example, be of the tapered roller type and which are mounted in suitable sockets indicated diagrammatically at 37, 38. A typical socket 38, as shown in FIG. 4, has a base portion 40 which is secured to the side wall of the frame by screws 41 and which mounts a carrier 42'which embraces the bearing 36. The carrier is fixed to the base by a first screw 43 which permits limited pivoting action and adjustably clamped in place by a clamping screw 44. It will suffice to say that roller sockets of the type shown not only serve to support the roller but serve to bias it into engagement with the associated cylinder while permitting throw off to facilitate cleaning.

The associated cylinder 25, formed of steel, has stub shafts 51 which are also journaled with respect to the press frame and which are positively rotated by the press drive by suitable means, not shown, but which are familiar to those skilled in the art. The ink rollers are driven by friction by reason of their contact with the surface of an associated cylinder.

In accordance with the present invention the ink rollers are provided with a layer of resilient insulating material about the outer surface and insulated from the press frame, with a high voltage difference applied between each ink roller and the associatedcylinder for setting up an intense electric field in the region of the intervening nip so that ink mist particles created at the nip and which have a casual charge are acted upon by the field for redepositing on the ink film on the surface of the ink roller and cylinder. Thus, turning to FIG. 2, the body 30 of the ink roller 26 a resilient insulating cover or layer 60 which extends the lengthof the roller body and which may, for example, be formed of resilient material having substantially the characteristics of the synthetic rubber known as Buna-N. In the case of an ink roller having a nominal diameter of 6 inches, the resilient covering 60 may have a radial thickness on the order of A to 1 inch. For the purpose of providing insulation between the stub shaft 31 and the bearing 35, an insulating collar 70, of nylon or the like, is provided which is of flanged cup shape having a cylindrical portion 71, a flange 72 and an end portion 73. At the opposite end of the ink roller a similar insulating collar 70 is provided having a cylindrical portion 71, a flange 72, and end 73 except that the end is provided with an axial opening 74 to provide access to a spring plunger 75 for establishing electrical contact with the body 30 of the roller. Stationarily mounted at the end of the plunger 75 is a contact 76 enclosed in a mount 77 of insulating material providing a terminal 78. The terminal 78 is connected to a source of high voltage 80, having a terminal 81, via an insulated line 82. The high voltage source 80 is preferably grounded to the frame of the press. The circuit from the ink cylinder 25 is completed either through the cylinder bearings or through a suitable wiper contact, with the ground connection being schematically indicated at 83.

While attention has been focused upon only a single ink cylinder 25 and single ink roller 26, it will be understood that the other ink cylinders and rollers in the system are similarly mounted and constructed and that the other roller bodies are similarly connected to the high voltage source 80, corresponding reference numerals, with the addition of subscript a, being applicable.

Prior to a discussion of the manner in which the ink mist is suppressed, attention may be given to FIG. 2a which shows, in greatly simplified form, the manner in which the mist particles are formed. In this figure the ink film, generally indicated at 90, is forceably separated into a roller film 91 and a cylinder film 92 at an exit nip 93. The inlet nip is indicated at 94. Because of the viscous nature of the ink, separation is accompanied by the formation of fine strands 95 which are quickly elongated to the point of rupture as indicated at 96 to eject one or more tiny droplets 97. It is to be noted that such droplets are not in the form of tangentially ejected spatter but are rather in the form of liberated particles of very small size, measured in terms of microns, sufficiently small to remain airborne for circulation with air currents within, and outside of, the press. When the rollers and cylinders are operated at a peripheral speed on the order of 1,000 to 3,000 feet per minute, the liberation of the particles occurs at a high rate to produce an objectionable mist level.

My observations show that the liberated ink particles, such as that indicated at 97 in FIG. 2a, acquire a casual electrical charge simply as a result of the liberation process. My observations show further that such casual charge is adequate to achieve control of the particles by the electric field which exists between the positively charged roller body 30 and the surface of the cylinder 25. The mechanism of charge acquisition, while by no means certain, may be explained as follows: While the resilient covering 60, which forms the surface of the roller 26, is of a material, for example, synthetic rubber of the Buna- N type, which is normally thought to be highly insulating and while the ink itself is nominally a non-conductor, both of the materials are in fact conductive to some slight degree, having a resistivity on the general order of to 10 ohms per centimeter cube, referred to as ohm-centimeters, which, under an impressed voltage on the order of 5 to 20 kilovolts, results in current flow to, and through, the ink and through the strands of ink 95 which are formed in the nip 93. When a strand ruptures, the resulting particles of ink will not be freed from electrical contact with both the surface of the ink roller and the ink cylinder at the same instant. Taking the particle 97 by way of example, it is freed first from contact with the ink roller and thereafter from contact with the ink cylinder so that it will tend to assume a charge of the polarity of the ink cylinder, in this case, negative. Due to induction a substantial charge may be acquired. Thus, in the case of the particle 97, the existence of the positive charge on the roller 26 immediately following rupture serves to attract electrons from the ink film 92 into the incipient particle 97 because of the fact that the particle, while still attached, is extended outwardly into the electrical field in the direction of the ink roller 26. Other particles, having final contact with the positively charged roller 26 just prior to liberation, will acquire a positive charge.

A further charging mechanism which may be applicable here is the self-charging of particles by triboelectric efiect. Thus the simple act of separating a small particle from a larger body of material tends to impart an electrical charge to the particle. In this connection it should be recalled that an ink is not perfectly homogeneous but is a mixture of particles of pigment in an insulating dielectric vehicle. When a small particle is liberated from the body of the ink the particle may have a higher or lower work function than the body of the ink. Where of a higher work function, a particle will acquire a negative charge. If the liberated particle has a lower work function, it will, upon separation, give up electrons and thereby acquire a positive electrical charge. Droplets separated from the thin ink films on rolls are likely to carry charges produced by separation of the charge double layer that exists in the ink near the liquid-roll interface or between pigment particles and the surrounding ink vehicle.

In any event the casual charge on the present ink particles, without use of corona discharge, is apparently of sufficient magnitude so that the particle is strongly affected by the electrical field which is produced in the region of the nip 93 between the ink rollers and the ink cylinder so that the particle is redeposited on the filmed surface of one of them before it can be blown out of the field by the strong convection currents which surround rapidly moving rollers and cylinders. In FIG. 2b, the electrical field, indicated at 100, is shown acting upon the negatively charged particle 97, causing it to be attracted along some reasonably short path 101 into contact with the ink film 91 on the roller 26. A positively charged particle, regardless of its source of charge, is similarly attracted to the film 92 on the cylinder 25. The effect in both cases is for the particle to rejoin the film prior to leaving the region of the field to prevent escape into the ambient air. The field 100 shown in FIG. 2b has been intentionally simplified. Use of rollers of different diameter will affect the symmetry as may the use of resilient surfacing materials of different dielectric con- Slant.

Tests with and without applied voltage show that, on the average, well over percent of the ink mist may be recaptured.

In practicing the present invention it is desirable to choose a resilient, insulating material 60 which is not a perfect insulator and it is desirable, further, for the layer to be of limited thickness to increase the field intensity in the nip region for a given applied voltage. A resistivity lying well within the range of 10 to 10 ohm-centimeters is preferred, with the voltage and thickness of insulating material being such as to give a small amount of leakage current but not exceeding a few milliamperes per roller, for example, not exceeding 1 to 5 milliamperes per roller. Where the resistivity is at the lower portion of the range or where the layer is too thin, the current drain from the high voltage supply 80, at a voltage level of 5 to 20 kilovolts, may be excessive, for example, at a level higher than 5 to 10 milliamperes per roller. Current drain is evidenced by the existence of a corona at the inlet nip 94 when the roller-cylinder couple is rotating and a corona discharge in both of the nips when the couple is at rest. Such corona discharge does not account for the charging of the particles at the exit nip since no corona is present at the exit nip under actual running conditions. With respect to the desired thickness of the layer 60, I have found a range of thickness from A to 1 inch to be optimum. A thickness greater than this substantially reduces the intensity of the field for a given applied voltage thereby reducing efficiency. A thickness of less than approximately one-fourth inch substantially increases current flow which wastes power as well as producing excessive corona discharge at the inlet nip, unless the voltage is decreased. It is a fortunate fact that conventional designs of ink rollers are surfaced with synthetic rubber such as Buna-N or material of roughly equivalent physical and electrical characteristics within a range of thickness on the order of A to 1 inch. As a result, simply by adding insulating collars 70 at the ends of the ink rollers in both new and existing designs of presses, the stage is set for application of high voltage to achieve the benefits of the present invention without addition of any further electrodes or other parts and without necessity for redesign or further modification.

It is to be particularly noted that in spite of the fact that a voltage at a level normally considered to be hazardous is applied to pre-existing and accessible elements in a printing press, the voltage is confined within the rollers which are protected, from one end to the other, by a heavy and pre-existing insulating layer. This is to be contrasted with use of wires or other auxiliary electrodes which must necessarily be exposed to inadvertent contact, which are subject to damage and which get in the way when the press mechanism must be cleaned or serviced. If desired even the ends of the rubber rollers, from the insulating cup 71 to the edge of the rubber layer 60, may be covered with insulation as indicated at 110 in FIG. 3 to make it impossible for the press maintenance or service personnel to come in contact with any portion of the conductors to which voltage is applied.

The present invention preferably employs inks having a re sistivity which is of the same order of magnitude as the rubber layer 60, in other words within the range of approximately to 10" ohm-centimeters which includes most of the commercial inks which are in widest usage, both black and in various colors, although operation is possible somewhat beyond these limits upon adjustment of the operating conditions. This specifically includes inks made up largely of carbon black where the insulation is provided by the mineral oil or other dielectric vehicle which surrounds each of the particles of pigment.

In canying out the invention, both direct voltage and alternating voltage have been found to produce the desired result. Direct voltage is preferred for black ink, with positive polarity being applied to the ink rollers. Such polarity is preferred since a higher voltage may be achieved in a given physical en vironment of the type set forth in the drawings with lower leakage current and consequently with less possibility of a flashover or breakdown when operating at extreme voltage levels. Good mist suppression can be achieved with most'inks using a direct voltage within the range of 5 to 30 kilovolts, with a voltage on the order of kilovolts being preferred. Such voltage corresponds to a leakage current, in a practical case, of no more than a few milliarnperes per nip, usually only a fraction of the current required by special corona-producing electrodes. Since the efficiency of mist suppression increases to some extent with voltage, a balance must be struck between efficiency of suppression and the leakage current which may be tolerated. A further advantage. of employing direct voltage is that efficiency of suppression is preserved at high peripheral speeds. Surface speeds encountered in a production press will usually lie between 1,500 and 3,000 feet per minute. At the higher end of this range the surface of the roller may move through an appreciable distance during the switching portion of the a c. cycle when the voltage is at a low level, so that some of the ink particles may escape, whereas direct voltage is maintained at a constant level.

Nevertheless it is desirable to employ alternating voltage when using inks which appear to be subject to pigment separation in the face of direct voltage. The latter phenomenon is the tendency of the component pigments of certain inks, in the film between the roller and cylinder, to migrate selectively to regions of positive and negative polarity so that the ink film on the roller, upon repeated rotation, tends to be of slightly difierentcomposition than the ink film on the cylinder. This is overcome by employing high voltage a-c. at conventional 60 Hz. frequency although even higher frequencies may be employed if desired. Good suppression efficiencies have been achieved for both colored and black inks at surface speeds within the range of 1,500 to 3,000 feet per minute using an a-c. voltage of 9.5 kilovolts r.m.s. corresponding to a peak voltage of approximately 13 kilovolts. As a convenience, the power supply 80, obtainable as a commercial unit, may be provided with a control knob for varying the voltage level as well as a switch to change the output voltage from direct to alternating to accommodate the difierent types of inks.

In the above discussion it has been assumed that the rollers and cylinders rotate in a particular direction. It is one of the features of the present invention that the suppressive effect is independent of the direction of rotation and that the press drive may be reversed as required for certain web runs. The comments relating to the action at nip 93 (FIG. 2a) upon reversal simply apply to the nip 94 which then becomes the exit nip.

In the above description the invention has been applied to a conventional arrangement in which the ink roller is of steel covered with a layer of rubber and where the rubber performs an insulating function as well as providing the necessary resilience. It will be apparent to one skilled in the art that it is the outer surface of the roller body which is significant in setting up the electric field. Consequently, the term body" as used herein is not limited to a massive metallic body of the type disclosed and, if desired, the roller may be constructed of non-metallic material with a thin metal shell immediately underlying the rubber layer and connected to the source of voltage to set up the field. It is conceivable also that rubber or other resilient material having a higher conductivity than that set forth above might be employed for the roller and that the necessary degree of insulation between the roller and cylinder may be achieved by applying to the surface of the cylinder a thin layer of dielectric material without departing from the teachings of the invention. While the term resilien has been used in connection with the rubber layer 60 on the ink roller, it will be understood that practicing the invention does not require use of a soft surfacing material and that the degree of resilience may be quite limited provided that intimate contact is achieved between the roller and the cylinder along the entire region of engagement.

It will also be understood that while the invention has particular application to a printing press for controlling the misting of ink, my teachings are applicable as an anti-misting measure wherever a film of liquid, of greater or less viscosity, having a tendency to produce mist, is conveyed by cooperating rollers rotating at a high peripheral speed. The term ink refers to such a liquid.

The anti misting arrangement is particularly suitable for use in assemblies having a large number of cooperating rollers or cylinders, particularly where they are reversible, in nested or folded relation, and where a portion of the nips would be inaccessible to conventional corona-type electrodes. In such an assembly all of the insulated rollers may be fed from a single power supply and inaccessibility to individual nips is of no consequence.

While the leakage current has been stated in the above discussion to be limited by the resistivity of the resilient insulating layer, it will be apparent that a series resistor may be used, for example in line 82, to limit the current to only a few milliarnperes per nip. Where there are a plurality of nips a separate line 82 is preferably run from the voltage supply 80 to each covered roller with adjustable, or adjusted, resistors in series with the individual lines. In this way the same voltage supply may be used for plurality of rollers having coverings of different thickness or resistivity to establish the desired level of leakage current in each of them. And while it is preferred to insulate the rollers by use of insulating collars 70, the entire socket 38 may instead be insulated from the press frame without departing from the invention.

The terms roller" and cylinder as used herein are synonymous to the extent that each is intended to denote a rotary, cylindrically shaped element in mutual, wetted surface contact with other similar elements for transmission of an ink film.

I claim as my invention:

1. In combination with a printing press having a press frame, a printing cylinder and an impression cylinder mounted on the press frame for printing on a cooperating moving web, and an roll cores directly without an air gap to another of said electrical energy supply terminals of substantially different electrical potential,

ink fountain supplying ink for the printing cylinder, ink train apparatus for transferring ink from the ink fountain to the printing cylinder comprising:

the first one of said intermediate transfer rolls in serial rolling contact additionally contacting said fountain roll in rolling relation and the last one of said intermediate transfer rolls in serial rolling contact additionally contacting said printing cylinder in rolling relation when ink supplied by said ink fountain is sequentially transferred from said fountain roll, serially through said intermediate transfer rolls, and to said printing cylinder.

2. The invention defined by claim 1 wherein said electrical energy supply terminals have an electrical potential difference in the approximate range of 10 to kilovolts, said intermeans rotatably mounting said transfer rolls on said press 15 m e di ate transfer rolls havin an electrical current am frame including means for electrically insulating said first b h f 1] d p d roll cores from said second roll cores so as to prevent a etween eac com a to core an 3 y a Short circuit relationship between Said first roll cores and acent second roll core which extends through a second roll Said second roll cores through Said press frame resilient, dielectric-coated surface and an ink film contacting first means electrically connecting each of said first roll Sald sufface and which develops elecmca} current m the cores directly without an air gap to one of said electrical approxlmat? range of 1 to 5 mmlamperes m Sald path by energy supply terminals and reason of said electrical potential difi'erence. second means electrically connecting each of said second

Claims (2)

1. In combination with a printing press having a press frame, a printing cylinder and an impression cylinder mounted on the press frame for printing on a cooperating moving web, and an ink fountain supplying ink for the printing cylinder, ink train apparatus for transferring ink from the ink fountain to the printing cylinder comprising: a fountain roll having a rotational axis with a fixed location relative to said press frame during printing and receiving ink at its cylindrical surface from said ink fountain, a high voltage electrical energy supply having terminals of substantially different electrical potential, a plurality of intermeDiate transfer rolls in serial rolling contact comprised of first rolls having metallic surfaces joined to respective electrically conducting first roll cores and of second rolls alternated with said first rolls and having resilient, dielectric-coated surfaces joined to respective electrically conducting second roll cores, means rotatably mounting said transfer rolls on said press frame including means for electrically insulating said first roll cores from said second roll cores so as to prevent a short circuit relationship between said first roll cores and said second roll cores through said press frame, first means electrically connecting each of said first roll cores directly without an air gap to one of said electrical energy supply terminals, and second means electrically connecting each of said second roll cores directly without an air gap to another of said electrical energy supply terminals of substantially different electrical potential, the first one of said intermediate transfer rolls in serial rolling contact additionally contacting said fountain roll in rolling relation and the last one of said intermediate transfer rolls in serial rolling contact additionally contacting said printing cylinder in rolling relation when ink supplied by said ink fountain is sequentially transferred from said fountain roll, serially through said intermediate transfer rolls, and to said printing cylinder.

2. The invention defined by claim 1 wherein said electrical energy supply terminals have an electrical potential difference in the approximate range of 10 to 15 kilovolts, said intermediate transfer rolls having an electrical current path between each combination of a first roll core and a serially adjacent second roll core which extends through a second roll resilient, dielectric-coated surface and an ink film contacting said surface and which develops an electrical current in the approximate range of 1 to 5 milliamperes in said path by reason of said electrical potential difference.

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