US3340802A - Electrostatic printing wherein screen carries powder between loading and printing points - Google Patents

Description

Sept. 12-, 1967 c. E. PILON 3,34

ELECTROSTATIC PRINTING WHEREIN SCREEN CARRIES POWDER BETWEEN LOADING AND PRINTING POINTS Filed May 19, 1966 o o o m o o c iv POWDER CLOUD I CHAMBER POWDEPL\ v T RESERVOIR MoToR 4 POWDER CLOUD K GENERATOR v v I1\ I ENTOR. CHARLES E. /L ON United States Patent 3,340,802 ELECTROSTATIC PRINTING WHEREIN SCREEN CARRIES POWDER BETWEEN LOADHJG AND PRINTING POINTS Charles E. Pilon, Berkeley, Calif., assignor to Electrostatic Printing Corporation of America, San Francisco, Calif., a corporation of California Filed May 19, 1966, Ser. No. 551,365 6 Claims. (Cl. 101114) ABSTRACT OF THE DISCLUSURE In an electrostatic printing system of the type wherein electroscopic powder particles are urged through the open apertures of an image screen into an electric field to be carried thereby through a substrate to form a powder image thereon, there is provided an improved arrangement wherein the powder particles are applied to the screen from a powder cloud chamber at a powder loading location. Between the powder loading location and a powder transferring or a printing location, toward which the screen is moved, there is provided a wiper or some other suitable means for removing the powder from the masked portions in the screen. At the printing, or powder transferring location, a wiper is applied to the back surface of the screen to mechanically actuate the screen whereby the powder particles on the opposite side of the screen are caused to leave the screen and move into the electric field to be carried in the form of a powder image to the receiving substrate. Provision is then made to make the wiper resistive or suitably conductive so that it functions to control the amount of powder which is removed from the screen into the electric field.

-scopic powder particles employs a screen having all openings blocked off except those which provide the desired image pattern. A conductive plate is spaced from the screen and the substrate upon which it is desired to print is inserted between the conductive plate and the screen surface. 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 of the screen to the substrate. The substrate is thereafter removed and the image is fixed to the substrate by well known means, such as heat.

Various arrangements have been contemplated and used for moving the powder particles through the openings of the screen into the electric field. One of these arrangements establishes a highly charged powder cloud which is directed to the back side of the screen by an electric field whereupon the powder particles move through the screen apertures which are open into the field between the substrate and the screen. Another means for applying powder is to use a brush to flick or rub the powder particles through the screen openings. Still another arrangement uses a metal plate on which the powder particles are deposited and which is brought to the back side of the screen. Upon the application of a voltage pulse between the plate and screen, the powder particles are caused to move toward the screen under the influence of the electric field which is established between the plate and the screen.

All of these arrangements have one common feature; the powder is brought to the screen and transferred there through to the image receiving substrate at the same location and substantially simultaneously. While for many applications this arrangement is satisfactory, for certain situations, which occur in the electrostatic printing process, this technique of simultaneously bringing powder to the screen while printing causes complications that preclude the designing of a simple and direct printing machine. For example, with rapidly moving conveyor lines or even where it is necessary to move the product relative to the screen, it is difficult to bring powder to the screen and pass it through the apertures, either sufficiently rapidly or with sufficient quantity during the interval within which printing is to occur. While such feeding equipment can be built, it is complicated and requires delicate adjustments and continual monitoring.

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

Still another object of this invention is the provision of a simplified system for loading powder on the screen of an electrostatic printing system at one location and transferring the powder to an image receiving object at another location.

These and other objects of this invention are achieved in an arrangement whereby powder is loaded onto an image screen over the apertures of the image screen at a place remote from the actual place of printing. The preloaded image screen and the object to be printed on are then brought into the proper relationship with one another at a printing location where the preloaded screen is effectively wiped to initiate the transfer of the powder, in the pattern of the screen image, to the receiving object.

The novel features that are considered characteristic of this invention are set forth with particularity 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 representation in elevation of an embodiment of the invention showing the preloading of powder at one side of the screen;

FIGURE 2 is an enlarged representation in elevation of the screen and wiper blades of FIGURE 1 which is shown to illustrate how the screen carries the powder;

FIGURE 3 is a schematic representation in elevation of an embodiment of the invention showing another exemplary arrangement for preloading powder; and

' FIGURE 4 is an isometric view of an arrangement for preloading a screen and printing therefrom in accordance with this invention.

Referring now to FIGURE 1, there may be seen a schematic representation in elevation of this invention. A conductive image screen 10, represented on edge, which has certain of its apertures blocked and others of its apertures open, in accordance with a pattern desired to be printed, is represented as moving in the directionof the arrows. The screen will be considered as having a printing side, the side which is opposite an object on which printing i to occur, and a back side, which is the side opposite the printing side. The printing side of the image screen passes adjacent a powder cloud chamber 12, from whence charged electroscopic powder particles within a dense powder cloud established within said chamber, by well known fluidizing means, are urged toward the screen 10 by virtue of an electric field which establishing potential 14 which may be connected between either the conductive base of the powder cloud chamber and the screen or between spaced wires (not shown) supported within the powder cloud chamber 12 and the screen. The amount of powder which is deposited on the screen from the powder cloud chamber may be controlled by controlling the amplitude of the potential 14 which establishes the electric field between the powder cloud chamber and the screen The powder particles adhere to the screen and fill the space between the screen, in the manner shown in FIG- URE 2, and remain there even though the screen is moving. As the screen moves out from under the powder cloud chamber it passes a wiper blade 16 which is positioned on the printing side of the image screen for the purpose of wiping off any of the excess powder which may extend too far beyond the surface of the screen or which may be on the non-image portion of the screen. It will be appreciated that powder which transfers to an object from the non-image portion of the screen can spoil the image. The wiper 16, by removing powder from all non-image screen areas, effectively insures that the powder that remains is only in the image areas. Thus, all that remains to be done is to transfer the remaining powder, in the form of the image areas, to a receiving object or substrate.

The image screen then proceeds from the wiper blade 16 to the printing region which is defined by the region between another preferably resistive wiper blade 18 positioned at the back side of the screen and the object 20 on which it is desired printing to occur. The object 20 is shown as circular, it may be a container upon whose surface it is desired to print. Since it is circular, in order to print over the surface it is necessary to rotate the container 20 at a speed substantially matching the speed of motion of the screen 10. An electric field is established between the screen and the object 20 by the use of a field establishing potential 22. A variable potential 24 is also connected between the wiper blade 18 and the screen 10.

The wiper blade 18 mechanically contacts the screen and the wiping action thereof on the screen together with the action of the electric field established between the screen and the object 20 has the effect of causing the powder particles which are held at the screen and bridging the openings of the screen to be released from the screen to be moved by the electric fiel-d onto the image receiving object 20. A further control on the amount of powder which moves through the apertures is established by the amplitude of the potential 24 which is applied to the wiper blade 18. This can hold at the blade 18 some of the powder which may be on the back side of the image screen. and otherwise would pass through the apertures into the electric field between the screen and the object 20. The screen passes out from the printing station and can be circulated, if so desired.

FIGURE 2 is an enlarged schematic drawing in elevation of a section of the screen 10 and the wiper blades 16 and 18, which are shown in order to assist in an understanding of this invention. In order to constitute the screen as the powder image forming electrodes cer tain areas of the screen have their apertures blocked or masked by a suitable insulating material 11, which may be nylon or Teflon or any other suitable insulator. Not only does this material fill the screen openings but it also extends a small distance away from the printing side of the screen wires. The reason for making the printing side of the screen the one in which the insulating material covers the screen wires and extends beyond is that it has been found that this eliminates multi-path activity which occurs with the more conductive powder particles which assume the charge of the surface with which they are in contact and then are repelled by the charge on the surface. The insulating material does not charge any powder particles which happen to come in contact therewith, and these powder particles remain on the surface of the material.

The charged powder particles 13 which are directed by the electric field established between the powder cloud chamber and the screen, to the screen, adhere to the screen wires, bridge their openings, and form one or more layers in the manner shown in the drawing. The wiper 16 scrapes the powder particles from the masking material of the screen, before the screen reaches the printing area, leaving the powder only covering the image areas of the screen. A container, not shown, may be employed at the base of the wiper 16 to retrieve the excess powder removed by this wiper blade.

At the printing area, the mechanical wipe of the screen wire by the wiper 18 dislodges the powder on the screen wires. Some of the powder is also dislodged by the action of the electric field established between the screen wires and the backplate or object being printed on. All of the dislodged powder is directed by the electric field which is established between the screen wires and the backplate or the object being printed on, to the Object. The potential 24 which is applied to the Wiper blade 18, may be applied across the wiper blade as shown in FIG- URE 1, or may be applied to a conductive wire 19, embedded in the resistive material of the wiper blade.

FIGURE 3 shows still another arrangement for preloading a screen 30. This time a brush 32 rotates so that its fibers can receive a load of powder from a powder reservoir 34. The brush continues to rotate until its fiber or bristles deposit the powder in the openings of an image screen 30. In order to control the amount of powder which is deposited and to retain the powder at the screen, a source of potential 36 is used to apply a bias voltage between the hub of the brush and the screen 30. As the screen continues to move away from the loading region, the image printing side of the screen is wiped clean by a wiper blade 38. The screen 30 moves into a printing position where this time the wiper is a roller 40 instead of a fixed wiper blade. The operation of the invention is the same as was previously described. The roller causes a mechanical agitation of the screen sufficient to loosen the powder particles at the screen openings 50 that under the influence of the electric field established between the object 42 and the screen by the field potential source 44 connected therebetween, the powder particles are deposited upon the object. A bias potential 46 may be connected between the roller and the screen, if the roller is made resistive for the purpose of determining how much of the powder which is loosened with the screen openings, is transferred to the receiving object 42.

FIGURE 4 shows an isometric view illustrative of an arrangement of the invention. The screen 50 is in webbed form and is stretched between rollers 54, 56. The motor 58 rotatably drives the roller 56 whereby the screen is continuously cycled by the rotation of the roller in the direction indicated by the arrows on the roller. A powder cloud generator 60 loads the openings of the screen with powder particles. The bias for transferring powder between the powder cloud chamber and the screen is provided by the bias source 62 applied between the generator 60 and a conductive plate 61 on the other side of the screen 50. A wiper blade 64 wipes the printing side of the screen 50 clean and any powder which is wiped off the screen is collected in a container 66.

The screen next moves to the printing position where a wiper blade 68 presses down on the screen to cause the powder particles which are in the image areas thereof to leave the screen and to be directed toward the object 72 which is being printed on. A source of bias potential 70 is connected between the wiper 68 and the screen 50. The electric field required for transferring the powder between the screen and the object 72 is provided by the bias source 74-, connected between the roller and the screen. After the screen moves through the printing station, it is turned around the roller 56 and the roller 54 to assume another powder load from the powder cloud chamber.

While the foregoing description of the embodiment of the invention is one wherein the screen moves past the powder loading station and then passes the powder image transfer station it should be realized that it is well within the scope of this invention as well as within the skill of those versed in the art to use a stationary screen with the powder loading arrangement being made movable together with the wiper at the printing position while the object being printed on is made stationary. Also, the screen need not be a continuous one which extends between the powder loading position and the powder image transfer position, but rather can be individual screen sections which are loaded and thereafter brought to the powder image transfer region for unloading.

While the wiper blade or roller at the printing position is shown with a potential applied between it and the screen for the purpose of assisting in controlling the amount of powder transferred at the image transfer location, it should be appreciated that it is also within the scope of this invention to omit the use of such potential and rely solely on the mechanical dislodgement of the powder by the wiper. The pressure applied by the wiper or roller on the screen at the image transfer location should not be such as will deform the screen but rather should be enough so that the relative motion between the wiper and the screen causes some movement of the individual strands of the screen such that the powder particles are dislodged solely mechanically or as a result of the combined mechanical and electrical forces.

There has accordingly been described and shown herein a novel, useful and simple arrangement for preloading the screen, and thereafter for transferring powder from the screen at a printing location.

What is claimed is:

1. In an electrostatic printing system, a conductive apertured screen having a printing side and a back side which is opposite said printing surface, a masking material blocking a portion of the screen apertures to leave the remaining unblocked apertures arranged in the form of a desired image, said masking material extending beyond said printing side of said screen, means establishing a screen loading location, means establishing 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 means establishing a screen loading location including a source of electroscopic owder particles, each of said powder particles being smaller than the apertures of said screen, means for transferring powder from said source onto said printing side of said screen, cleaning wiper blade means positioned between said screen loading and unloading position for removing powder particles deposited over said screen masking material at said screen loading location, said means establishing said screen un loading location including means for establishing an electric field between said screen and an image receiving object positioned adjacent said printing side of said screen, and printing wiper means in contact with the back side of said screen, said printing wiper means mechanically agitating and Wiping the back side of said screen for mechanically dislodging the powder particles on said screen into the electric field between said screen and an image receiving object to be moved toward said image receiving object under the influence of said electric field.

2. In an electrostatic printing system a recited in claim 1 wherein said printing wiper means comprises a wiper blade made of resistive material and there is included a source of potential and means connecting said source of potential between said screen and said wiper blade.

3. In an electrostatic printing system as recited in claim 1 wherein said printing wiper means comprises a roller in frictional engagement with said screen, and there is provided a source of electric potential and means for connecting said source of electric potential between said roller and said screen.

4. In an electrostatic printing system as recited in claim 1 wherein said printing 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.

5. 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 powder cloud forming means and said screen for establishing an electric field there between to transfer powder particles from said means for establishing a cloud of powder particles to said screen.

6. In an electrostatic printing system as recited in claim 1 wherein said means for transferring powder particles from said source onto said screen includes a rotatable brush having bristles extending from a conductive hub and being positioned for bringing its bristles into contact with the surface of said screen, a source of electric potenital, and means for connecting said source of electric potential between the hub of said brush and said screen for controlling the amount of powder to be transferred between said brush and said screen.

References Cited UNITED STATES PATENTS 2,824,813 2/1958 Fauser et al. 2,959,153 11/1960 Hider. 2,966,429 12/ 1960 Darrel et al. 3,081,698 3/1963 Childress et al. 3,152,012 10/ 1964 Schaffert. 3,220,833 11/ 1965 McFarlone. 3,245,341 4/1966 Childress et al. 3,253,540 5/1966 Lusher. 3,261,284 7/1966 Lynott et al.

ROBERT E. PULFREY, Primary Examiner,

E. S. BURR, Assistant Examiner.

Claims (1)

1. IN AN ELECTROSTATIC PRINTING SYSTEM, A CONDUCTIVE APERTURED SCREEN HAVING A PRINTING SIDE AND A BACK SIDE WHICH IS OPPOSITE SAID PRINTING SURFACE, A MASKING MATERIAL BLOCKING A PORTION OF THE SCREEN APERTURES TO LEAVE THE REMAINING UNBLOCKED APERTURE ARRANGED IN THE FORM OF A DESIRED IMAGE, SAID MASKING MATERIAL EXTENDING BEYOND SAID PRINTING SIDE OF SAID SCREEN, MEANS ESTABLISHING A SCREEN LOADING LOCATION, MEANS ESTABLISHING A SCREEN UNTION, MEANS FOR RELATIVELY MOVING SAID SCREEN BETWEEN SAID SCREEN LOADING LOCATION AND SAID SCREEN UNLOADING LOCATION, SAID MEANS ESTABLISHING A 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 SAID PRINTING SIDE OF SAID SCREEN, CLEANING WIPER BLADE MEANS POSITIONED BETWEEN SAID SCREEN LOADING AND UNLOADING POSITION FOR REMOVING POWDER PARTICLES DEPOSITED OVER SAID SCREEN MASKING MATERIAL AT SAID SCREEN LOADING LOCATION, SAID MEANS ESTABLISHING SAID SCREEN UNLOADING LOCATION INCLUDING MEANS FOR ESTABLISHING AN ELECTRIC FIELD BETWEEN SAID SCREEN AND AN IMAGE RECEIVING OBJECT POSITIONED ADJACENT SAID PRINTING SIDE OF SAID SCREEN, AND PRINTING WIPER MEANS IN CONTACT WITH THE BACK SIDE OF SAID SCREEN, SAID PRINTING WIPER MEANS MECHANICALLY AGITATING AND WIPING THE BACK SIDE OF SAID SCREEN FOR MECHANICALLY DISLODGING THE POWDER PARTICLES ON SAID SCREEN INTO THE ELECTRIC FIELD BETWEEN SAID SCREEN AND AN IMAGE RECEIVING OBJECT TO BE MOVED TOWARD SAID IMAGE RECEIVING OBJECT UNDER THE INFLUENCE OF SAID ELECTRIC FIELD.

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