US3340803A - Electrostatic printing with powder applied to screen on printing side - Google Patents

Description

E /B 52 VACUUM POTENTSAL su pw SOURCE souRcE.

SOURCE Sep 1967 c.o. CHILDRESS ETAL 3,340,303 ELECTROSTATIC PRINTING WITH POWDER APPLIED TO SCREEN ON PRINTING SIDE Filed- May 19, 1966 2 Sheets-Sheefl SOURCE OF POTENT\AL 4 SOURCE OF POTENTlAL.

C. O. CHILDRESS ETAL Sept. 12, 1967 3,340,803

ELECTROSTATIC PRINTING WITH POWDER APPLIED TO SCREEN ON PRINTING SIDE 2 Sheets-Sheet 5;

Filed May 19, 1966 VACLU UM 50M RCE SOU RCE OF POTEN'UAL FLUID\ZED POWDER SLAPMTY FLUID)ZED POWDER 5U PPLY sou RCE OF POTENT\AL l THLRD SOURCE OF POTENTMNL SEcoH D sou (26E OF POTENTlAL F R 5T 50M PCE OF- POTENT \AL POTEN'HA 6 x 5 8 m y R OR E 0 D N I?! T NM. R mcA w 0 A E 4. FL DN m om MM I ET i 5y 00 B 5 O m .r am O O %P l 4 WY fi a SP n u rmwww .w .K U 0 mp Patented Sept. 12, 1967 ABSTRACT OF THE DISCLOSURE p In an electrostatic printing system using an image screen through which powder particles, smaller than the openings of the screen are passed into an electric field which carries them in the form of a powder image to a substrate, there is provided a means, at a powder loading location, for applying powder particles to the back side of the screen. The powder particles adhere to the screen in sufiicient number to provide an excellent print when at a printing location, spaced from the powder applying location, a wiper blade applied to the backside of the screen urges the powder particles through the screen openings into an electric field to be moved to the substrate spaced from the screen. The wiper blade may be made of a material so that it may be suitably electrically biased whereby it can control the amount of powder which is transferred from the mechanically wiped screen.

This invention relates to systems for printing by introducing electroscopic powder particles into an electric field, and more particularly to improvements therein.

One of the present systems for printing with electroscopic powder particles employs a screen having all openings blocked off except those which provide a desired image pattern. A conductive plate is spaced from this screen and the substrate upon which it is desired to print, is inserted between the conductive plate and the screen surface. The conductive plate may be eliminated if the substrate is sufficiently conductive. A potential for the purpose of establishing an electric field is established between the screen and the conductive plate. Electroscopic powder particles are urged through the screen apertures into the field between the screen and the conductive plate, whereupon they are carried in the pattern of the image apertures of the screen to the substrate. The substrate is thereafter removed and the image is fixed to' the substrate by well known means, suchas heatQThis general technique is described and claimed in a patent to Childress et al., Patent No. 3,081,698.

Various arrangements have been contemplated and used for moving powder particles through the openings of the screen into the electric field. In one of these, powder is applied to a brush which is rubbed against the screen to flick the powder particles through the screen openings. Another technique establishes a highly charged powder cloud which is directed at the backside of the screen by an electric field having a sufiicient energy to cause the powder particles to move through the screen apertures which open into the field between the screen and the substrate. It has been found desirable to print on cylindrical objects as well as on relatively flat surfaces. When a fiat screen is used as the image forming member for a cylindrical object, it is necssary to confine the area of powder transfer to a relatively narrow portion of the screen adjacent to which the surface of the cylindrical object, which is caused to revolve, is in closest proximity. In this manner, a substantially parallel electric field can be established between the two proximal surface areas of the screen and cylinder. It has been found difficult to confine the action of the brush or the action of the air suspended particles to such a restricted area of the screen, particularly if high printing speeds are to be achieved.

An object of this invention is the provision of a novel powder loading and powder transferring system in an electrostatic printing system of the type described.

Still another object of this invention is the provision of a powder loading and powder transferring system in a system of the type described, suitable for high speed printing on cylindrical objects.

Yet another object of the present invention is the provision of a powder loading and powder transferring arrangement in a system of the type described, which conserves the amount of powder which is used.

These and other objects of the present invention are achieved in an arrangement whereby a thin coating of electroscopic powder particles is preloaded or caused to adhere electrostatically to the rear surface of an image forming screen. The powder particles can then be caused to pass through the image openings by passing a narrow wiper blade along the rear surface of the screen, as a result of which powder penetration through the screen is confined substantially to the contact area of the blade. Such a blade may be positioned to move along the rear of the screen synchronously with the closest portion of the cylinder which is being printed. This cylinder can be transported and rotated along the flat surface of the image screen in a manner to have a minimal relative surface velocity in the area of powder transfer. Thus, the screen may be stationary, its back surface may be loaded with powder particles, and thereafter the cylinder is moved linearly and rotated adjacent the front surface of the screen with a blade being simultaneously moved along the back surface of the screen. Alternatively, the screen may be moved with its back surface being preloaded with powder at one location, and at a second location a wiper blade is positioned at the backside of the screen with the rotating cylindrical member being positioned adjacent the front side of the screen and opposite the blade location.

The novel features that are considered characteristic of this invention are set forth with particularly. in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic illustration of an embodiment of the invention;

FIGURE 2 is an enlarged representation of the section of FIGURE 1 at which the Wiper blade is positioned to illustrate the operation of the blade at this location;

FIGURE 3 illustrates another arrangement for preloading the screen in accordance with this invention;

FIGURE 4 shows another arrangement for cleaning the screen front surface while its back surface is being loaded; and

FIGURE 5 illustrates an arrangement for employing the invention in a continuous belt printing arrangement.

FIGURE 1 is a schematic representation of am embodimentof the invention using a moving screen with the powder preloading apparatus and the object receiving the printing moving synchronously so that it and the screen are relatively stationary. It should be appreciated however that this arrangement is to be considered as illustrative since it is believed obvious to maintain the screen stationary and move the powder loading apparatus and the powder image receiving object past the screen, where the circumstances of manufacture require this kind of an operation. In accordance with this invention, an image aperture screen 10 moves in the direction shown by the arrows so that initially, a portion thereof passes adjacent a rear loading electrode 12. This electrode comprises a conductive plate which may be spaced on the order of one-half inch from the screen 10. A source of fluidized powder 14, or powder supply source applies the electroscopic powder particles to be employed in the printing through a nozzle 16 to the space between the rear loading elect-rode 12 and the screen 10. A source of potential 18 is connected with its negative terminal to the rear loading electrode and its positive terminal grounded. The screen is also connected to ground. As a result, an electric field is established between the electrode 12 and the screen whereby powder in the fluidized powder stream directed downward by the nozzle 16 between the screen and the electrode, is directed onto the screen. The location at which the electrode 12 directs powder onto the screen may be considered as apowder loading location.

Any powder in the fluidized stream which passes completely between the electrode 12 and the screen 10 and is not captured by the screen, is caught in a container 20 which is below the screen. As the screen continues to move in the direction of the arrow, the front surface thereof, which is the surface opposite to the one on which powder is loaded, passes by a rotating belt 22, which serves the function of wiping powder from the front surface of the screen that may have penetrated the image openings and been deposited on the front surface of the screen. The belt 22 which is supported on spaced rollers 24, 26 passes by the nozzle 28 which is connected to a vacuum source 30 whereby the powder picked up by the belt 22 is cleaned therefrom.

, The screen next moves to the location of a wiper blade 32, which serves the function of wiping the powder deposited on the back surface of the screen into and through the openings in the screen. Positione directly opposite the wiper blade is the object upon which it is desired t print. Here the cylindrical object 34 is shown as a bottle which is rotated in the direction of the curved arrow by any suitable means, to maintain a substantially zero relative motion with respect to the screen at the location of the wiper 32.

The wiper blade may be fabricated from a number of materials, depending upon the density of image deposit required and the nature of the electroscopic powder. For example, blades have been made of a thin ceramic material and also of a plastic. In some cases the wiper blade has been in effect a very narrow brush. It is desirable to apply a potential to the wiper 32. This may be done by embedding conductive material such as a wire 36, represented by the dotted lines, in the relatively nonconducting blade near the end closest to the screen. This wire is connected to another source of potential 38, which has its negative terminal connected to the wire 36 and the positive terminal grounded. Assuming that the object 34 which is receiving the printing is conductive, in whch event the conductive plate behind it is omitted, connection is made thereto from the positive terminal of a potential source 40, which has its negative terminal grounded. By way of illustration, but not by way of .a limitation, in an embodiment of the invention which was built and operated, the source of potential 18 was on the order of 12 kilovolts, the source of potential 38 was on the order of kilovolts, and the source of potential 40 was on the order of +2 kilovolts.

The method of loading the rear screen surface which is shown in FIGURE 1 is particularly effective when a high density flow of powder is applied between the rear of the screen and the electrode 12. High powder density causes the powder coating to quickly bridge the screen openings so that a minimal amount of powder penetrates these openings ahead of the wiper blade station. This may be seen by referring to FIGURE 2 which is an enlarged cross-sectional representation of the screen showing how the powder particles 44 bridge the screen openings. The powder also adheres to the back of the screen in the region where the openings are blocked as by the member 46.

Another method for filling the openings of the screen would be to extend the nozzle 16 so that it incorporates the electrode 12 instead of terminating in the space between the electrode and the screen, whereby a low pressure spray nozzle with the electrode therein for establishing a suitable field, directs powder at the rear screen surface.

FIGURE 2 also shows the operation of the wiper blade 32 to push the powder particles from the back surface of the screen through the openings of the screen whereby the electric field established between the screen and the object 34 carries these powder particles to the surface of the cylindrical object 34.

When the wiper blade is of a rigid material, the amount of powder transferred through the screen is somewhat dependent upon the shape of the leading edge of the blade and the width of the blade. By way of illustration, and not by way of limitation, a typical blade, of an inch wide, has its end terminating at the screen in the shape shown in FIGURE 2. The leading edge of the blade clears the screen by approximately ,434 of an inch.

FIGURE 3 shows another arrangement for coating the back side of the screen, in accordance with this invention. The coating arrangement comprises a housing 50, with insulating walls except that a conductive plate 52 is mounted in the back wall thereof. This conductive plate is connected to the negative terminal of a first source of potential whose positive terminal is grounded. Spaced from the conductive plate 52 are conductive wires or rods 56 which may be supported within the opposite walls of the housing to form an open grid. A fluidized powder supply source 58 furnishes fluidized powder to the region between the conductive plate 52 and the wires 56. A sec ond source of potential 60 has its negative terminal Connected to all of the wires in the grid 56 and its positive terminal grounded. The potential from the source 60 is less negative than the potential from the source 54, as a result of which powder particles in the fluidized powder supply which are directed between the plate 52 and the grid 56, are directed through the grid 56 at the periphery of a roller 62 which is revolved against the surface of the screen 10. As a matter of fact, the roller is rotatably supported, by any suitable means, not shown, with its periphery in frictional engagement with the surface of the screen so that as the screen moves past the roller it causes it to rotate.

The roller is either entirely made of resistive material, or has its surface made of a material which is sufiiciently resistive to maintain a potential difference between it and the screen with which it is in contact. The inner surface of the roller is connected with the negative terminal of a third source of potential, which is less negative than the second source of potential, and which source of potential has its positive terminal grounded. The roller may be made of any plastic material or semiconductor material, for example. The use of a material for the roller, or a coating for the roller, which has a very limited con ductivity, makes it possible to establish and maintain a suitable potential diflerence between the roller and screen so that the powder layer which is caused to adhere to the surface of the roller by passing through the grid 56, is thereafter caused to transfer from the roller to the rear screen surface under the influence of the potential from the third source. Thereafter, the operation of the system is as has been described in connection with FIGURE 1 and FIGURE 2. Accordingly, there has been omitted the showing in this drawing of the wiper blade and the cleaning apparatus at the front side of the screen.

As described above, during the process of coating the rear of the screen, some of the powder may penetrate the image openings and, in the absence of any further transferring fields at this location, this powder re-attaches itself to the front screen surface surrounding the image areas, from which location powder particles can subsequently transfer to the'image receiving object. As shown in FIGURE 1, a wiping surface is provided for the front side of the screen which is located between the powder loading station and the powder transfer station of the screen. Another effective means for preventing powder from adhering to the front face of the screen is shown in FIGURE 4. Here the structure for loading the back sur face of the screen which is shown is the same as that shown in FIGURE 1 and therefore the same reference numerals are applied.

On the front side of the screen opposite to the powder loading location, there is positioned a grid electrode 70, behind which there is placed a nozzle 72 which is coupled to a vacuum source 74. The grid electrode is biased positive from a source of potential 76 which has its negative terminal grounded. Thus, any powder particles which find their way through the screen openings, before the screen openings are clogged or blocked by the powder accumulation, are attracted by the electric field between the screen and the grid electrode 70. These powder particles therefore do not redeposit on the front surface of the screen but are drawn into the vacuum source 74. Alternative to the stationary grid 70, a conductive belt may be employed which may be biased similarly to attract thereto powder particles which find their way to the openings in the screen. The belt can be made rotatable to pass by a vacuuming device, in the manner shown in FIGURE 1.

FIGURE 5 illustrates a system in accordance with this invention which may be used with a continuous belt printer. The image screen 80 has the form of a continuous loop which is supported between two spaced rollers respectively 82, 84. Roller 84 is driven rotatably by a motor 86. The belt 80 is connected to ground. The powder loading on the rear surface of the screen may be accomplished by any of the arrangements described previously. However, still another arrangement is shown here. A hollow container 88 is positioned with one open side adjacent the backside of the image electrode 80. A porous metal plate 90 is supported spaced from the bottom side of the container 88 and air from a low, positive pressure air supply source 92 is fed between the bottom side of the container 88 and the porous plate 90. A powder charge is deposited over the porous plate 90 so that under the influence of the low, positive pressure air, the powder is effectively fluidized.

Within the container and adjacent the porous metal plate there is supported rotatably a brush 94, which is rotated by any suit-able means, not shown. The ends of the brush opposite the fluidized powder bed rub against the wires of a grid 96, which establish a powder cloud between the grid wires and the screen 80. The grid wires are connected to the negative terminal of a source of potential 98 which has its positive terminal grounded. The porous plate 90 is connected to the negative terminal of a source of potential 100, whose positive terminal is grounded. The relative amplitudes of these potential sources is such that the powder particles are urged from the plate 90 toward the grid 96 and by the field established between the grid and the screen, toward the screen. As the screen continues to rotate past the powder loading station, it is brought to the powder transfer station where the wiper blade 102 is positioned with one edge in contact with the screen rear surface. A source of potential 104 may be connected to the wiper blade 102.

The image receiving substrate here is exemplified by a web of paper 106, which is fed from a pay out roll 108, over a rotating conductive cylinder 110, which serves to move the surface of the paper close to the front side of the screen opposite the wiper blade 102, and thereafter through a powder fixing station 112, which fixes the powder by heat, and onto a take up roll 114. The positive terminal of a potential source 116 is connected to the conductive roller 110 and its negative terminal is connected to ground. As a result, the potential source 116 establishes an electric field between the conductive roller and the image screen whereby powder particles which are forced into the field by the wiper blade 102 are transferred across to the paper. The paper is moved synchronously with the surface of the belt so that the relative motion at the nip of the roller and the image screen, at the location of the wiper blade, is substantially zero.

Any excess powder on the back surface of the screen, which is not forced through the holes in the screen, falls as a result of being loosened by the wiper blade into a trough 118, which leads back to the porous plate 90, whereby the excess powder is returned to the fluidized bed to be re-used.

It should be appreciated that while the arrangement shown is one wherein a printing is made from a flat screen onto an object having a cylindrical surface, the advantages of this system are also to be had when printing from a curved screen onto a flat object, and in some cases from a flat screen onto a flat surface. This arises from the fact that in the use of a continuous belt screen for printing onto a paper web, it is often difficult to maintain a fixed gap between the image electrode belt and the paper web. By using a wipe transfer, which is limited to a very narrow transverse section of a moving image belt, it is possible to feed the paper around a cylindrical roller, as shown in FIGURE 5, to more easily maintain a substantially uniform gap in the area where the powder transfer actually takes place.

Another advantage of the above-described wipe transfer system, as this system may be designated, is the elimination of the ghosting problem. Ghosts are image density variations caused by depletion of the powdered ink on the brush or other powder applying mechanisms, which arise as a result of there being image areas of great demand which are incompletely satisfied and as a result of the accumulation of ink in areas of lesser demand. Such problems are common to roller inking systems in wet ink as well as in dry ink printing. The wipe transfer method insures that all image areas have been loaded with suflicient powder for a full density image. The metering action of the wipe blade is governed by the shape of its edge as well as the potential applied thereto, and is largely unaffected by excess powder on the screen which falls away from the blade so that excess powder is not accumulated.

What is claimed is:

1. In an electrostatic system a conductive screen having apertures therethrough arranged in the form of a desired image, a screen loading location, a screen unloading location displaced from said screen loading location, means for relatively moving said screen between said screen loading location and said screen unloading location, said screen having a back side to which powder particles are applied and an opposite side, said screen loading location including a source of electroscopic pow der particles, each of said powder particles being smaller than the apertures of said screen, means for. transferring powder from said source onto the back side of said screen, means positioned between the screen loading and unloading locations for removing any powder from the opposite side of said screen, said screen unloading location including means for establishing an electric field between said screen and an image receiving object posi tioned adjacent one surface of said screen, and wiper means in contact with the surface of the back side of said screen opposite the position of said object, said wiper means wiping the surface of said screen for pushing the powder particles on said screen through the apertures of said screen into the electric field between said screen and object to be moved toward said image receiving object under the influence of said electric field.

2. In an electrostatic printing system as recited in claim 1 wherein said wiping means comprises a wiper blade made of resistive material and there is included a source of potential and means connecting said source )f potential between said screen and said wiper blade for controlling the amount of powder particles which are .ransferred from said screen to said object.

3. In an electrostatic printing system as recited in claim 1 wherein said wiper means comprises a wiper blade having a conductor adjacent'the tip thereof, said conductor being coextensive with said screen, said means for establishing an electric field between said screen and said object includes a source of electric potential, and means for connecting said conductor and said object to said source of electric potential.

4. Apparatus as recited in claim 1 wherein said source of electroscopic powder particles comprises means for generating a cloud of electroscopic powder particles positioned at one side of said screen, and said means for transferring powder particles from said source onto said screen includes a source of electric potential, and means for connecting said source of electric potential between said cloud forming means and said screen for establishing an electric field therebetween to transfer powder particles from said means for establishing a cloud of powder particles to said screen.

5. In an electrostatic printing system as recited in claim 1 wherein there is positioned between said screen loading location and said screen unloading location and on the same side of said screen as said image receiving object means for removing powder particles on that side of said screen including means for wiping said screen.

6. In an electrostatic printing system as recited in claim 1 wherein there is positioned at said screen loading location adjacent the surface of said screen opposite to the one 'onto which powder is loaded electrode means, means for establishing a' potential between said screen and said electrode means for directing toward said electrode means powder particles which have passed through the apertures of said screen, and means for removing said powder particles from said electrode means.

7. In an electrostatic printing system as recited in claim 1 wherein said means for transferring powder from said source onto said screen includes roller means rotatably supported with a portion of its periphery in contact with said screen, electrode means positioned spaced from another portion of the periphery of said roller means, means for introducing a cloud of electroscopic powder particles from said source between said electrode means and said roller means periphery, means for establishing an electric field between said electrode means and said roller means periphery for transferring powder particles from said powder cloud to said roller means periphery, and means for establishing a potential difference across said roller means for transferring powder particles from said roller means to said screen.

References Cited UNITED STATES PATENTS 2,484,671 10/1949 Bauman 101 3,220,833 11/1965 McFarane 101 3,245,341 4/1966 Childress et a1 101--122 3,251,685 5/1966 Bickmore 101 3,273,496 9/1966 Melmon 101--144 ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner.

Claims (1)

1. IN AN ELECTROSTATIC SYSTEM A CONDUCTIVE SCREEN HAVING APERTURES THERETHROUGH ARRANGED IN THE FORM OF A DESIRED IMAGE, A SCREEN LOADING LOCATION, A SCREEN UNLOADING LOCATION DISPLACED FROM SAID SCREEN LOADING LOCATION, MEANS FOR RELATIVELY MOVING SAID SCREEN BETWEEN SAID SCREEN LOADING LOCATION AND SAID SCREEN UNLOADING LOCATION, SAID SCREEN HAVING A BACK SIDE TO WHICH POWDER PARTICLES ARE APPLIED AND AN OPPOSITE SIDE, SAID SCREEN LOADING LOCATION INCLUDING A SOURCE OF ELECTROSCOPIC POWDER PARTICLES, EACH OF SAID POWDER PARTICLES BEING SMALLER THAN THE APERTURES OF SAID SCREEN, MEANS FOR TRANSFERRING POWDER FROM SAID SOURCE ONTO THE BACK SIDE OF SAID SCREEN, MEANS POSITIONED BETWEEN THE SCREEN LOADING AND UNLOADING LOCATIONS FOR REMOVING ANY POWDER FROM THE OPPOSITE SIDE OF SAID SCREEN, SAID SCREEN UNLOADING LOCATION INCLUDING MEANS FOR ESTABLISHING AN ELECTRIC FIELD BETWEEN SAID SCREEN AND AN IMAGE RECEIVING OBJECT POSITIONED ADJACENT ONE SURFACE OF SAID SCREEN, AND WIPER MEANS IN CONTACT WITH THE SURFAE OF THE BACK SIDE OF SAID SCREEN OPPOSITE THE POSITION OF SAID OBJECT, SAID WIPER MEANS WIPING THE SURFACE OF SAID SCREEN FOR PUSHING THE POWDER PARTICLES ON SAID SCREEN THROUGH THE APERTURES OF SAID SCREEN INTO THE ELECTRIC FIELD BETWEEN SAID SCREEN AND OBJECT TO BE MOVED TOWARDS SAID IMAGE RECEIVING OBJECT UNDER THE INFLUENCE OF SAID ELECTRIC FIELD.

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