US3382796A - Apparatus for continuous electrostatic screen printing with density control - Google Patents

Description

May 14, 1968 J, JAVORIK ET AL 3,382,796

APPARATUS FOR CONTINUOUS ELECTROSTATIC SCREEN PRINTING WITH DENSITY CONTROL Original Filed Nov. 5, 1964 V39 Fuel (.UEQK SPRNG SPR MG INVENTORS LAsno iJnvoRm 3- EDuJmRD D. H |GG\NS ATTORNEYS United States Patent 3,382,796 APPARATUS FOR CONTINUOUS ELECTROSTATIC SCREEN PRlNTiNG WITH DENSITY CONTROL Laszlo J. Javon'k, Chicago, and Edward D. Higgins, Palos Heights, 11]., assignors to Continental Can Company, Inc., New York, N.Y., a corporation of New York Continuation of application Ser. No. 409,213, Nov. 5, 1964. This application Jan. 10, 1967, Ser. No. 609,275 6 Claims. (Cl. 101-122) work ABSTRACT OF THE DESCLGSURE Cross-reference to related application This is a continuation of application Ser. No. 409,213, filed Nov. 5, 1964, and now abandoned.

This invention relates to printing and more particularly to apparatus for continuous electrostatic screen printing.

It is known in the prior art to utilize toner particles for printing upon a substrate. In one method the toner particles become electrically charged by direct contact with a conductive screen While being brushed through the screen, While in another method the particles are triboelectrically charged by contact with conductive carrier particles to which the toner then adheres, after which the combined carrier and toner particles are accelerated in an electric field and caused to strike the stencil screen whereby the momenta of the toner particles carry the toner particles through apertures in the stencil screen and into contact with the substrate being printed. Thus, the toner particles are either charged by metallic contact with the stencil screen or by the triboelectric effect of the carrier particles. Certain advantages are inherent in each of these known printing systems. However, certain disadvantages are also inherent therein, such as the fact that neither system can provide the toner particles with enough charge, thus enough speed, which is essential for high speed electrostatic printing. Also, in the first system, it is even possible that a certain amount of toner does not get in touch with the stencil screen at all because it is mechanically pushed through apertures in the stencil screen without touching the wire. Such toner particles will not show proper electrostatic adhesion to the substrate and may not be securely deposited on the correct parts of the pattern being printed.

It is an object of this invention to provide apparatus for charging toner particles to a high enough level by a corona discharge so as to propel the toner particles with high velocity through a stencil screen.

Another object of this invention is to provide apparatus for successive cleaning and ion-repulsion operations so as to produce a dark, good quality electrostatic print with high speed.

Another object of this invention is to provide apparatus for producing an electrostatic ionic precharge of the sub strate to be printed.

A further object of this invention is to provide a continuous electrostatic screen printing machine wherein new and novel means are utilized for cloudizing toner particles, i.e., forming a suspension of toner particles in air, and new and novel means are provided for automatically controlling the amount of toner particles used in a printing operation.

A further object of this invention is to provide such a printing machine with new and novel apparatus for cleaning the stencil screen between successive prints.

A further object of this invention is to provide apparatus for use in an electrostatic printing device, the apparatus comprising a toner reservoir providing a supply of toner particles, means for agitating the supply of toner particles, corona electrode means, and means for causing the agitated toner particles to move past the corona discharge means.

Another object of this invention is to provide apparatus, of the type described above, wherein the means for agitating the supply of toner comprises sensing means for sensing toner density adjacent the corona electrode means, and means connecting the sensing means to the means for agitating the supply of toner to vary the amount of toner moving past the corona discharge means.

A further object of this invention is to provide apparatus, of the type described above, wherein the printing device prints upon a substrate, the means for agitating the supply of toner comprising sensing means for sensing the darkness of a printed portion of the substrate, and means for connecting the sensing means to the means for agitating the supply of toner to vary the amount of toner moving past the corona discharge means.

Another object of this invention is to provide an electrostatic screen printing machine of the type utilizing toner particles for printing upon a substrate, having an endless stencil screen disposed to form a loop, feed means for introducing toner particles to the interior of the loop between the corona pins and screen which is adjacent to the substrate and in the direction of the substrate, means for positioning the substrate adjacent to the exterior of the loop on the side opposite thereof from the feed means, and means for accelerating the toner particles toward the stencil screen, whereby some of the toner particles are stopped by the stencil screen and others of the particles pass through apertures in the stencil screen and impinge upon the substrate.

Another object of this invention is to provide an electrostatic screen printing machine, of the type described above, wherein the means for accelerating the toner particles can be of several forms, the most essential of which consists of a multi pin corona unit positioned within the screen loop with voltage applied between the corona unit and the means for positioning the substrate with the print ing screen and substrate between the corona unit and means for positioning the substrate. This physical relationship enables the corona discharge to charge the toner particles between the corona unit and the screen and also to generate voltage in the screen which helps propel the toner particles through the screen and onto the substrate.

The voltage on the screen may also be controlled to a definite value rather than accepting the value generated by the electric charge picked up by the screen. In this solution a separate power supply is applied between the screen and the means for positioning the substrate as shown on FIGURE 1.

Another object of this invention is to provide cleaning means which are disposed adjacent to the stencil screen for dislodging toner particles from the stencil screen, wherein the cleaning means comprises brush means disposed within the loop and in contact with the stencil screen, and air blast means and vacuum means disposed on the return section of the loop whereby toner particles dislodged by the brush means are blown from the stencil screen by the air blast means and collected by the vacuum means.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claimed subject matter and the several views illustrated in the accompanying drawing.

In the drawing:

FIGURE 1 is a diagrammatical longitudinal cross-section of an electrostatic printing machine embodying the present invention.

FIGURE 2 is a diagrammatical view, with portions broken away, and illustrates preferred apparatus for producing a cloud of toner particles and ionizing the toner particles by utilizing a corona discharge.

FIGURE 3 is a diagrammatical view of an electronic circuit utilized in the present invention.

Referring to the drawing in detail, a continuous stencil screen is disposed so as to form a loop about a pair of spaced rollers 11 and 12. One of the rollers, such as the roller 11, is suitably mounted on a shaft 13 which is driven by drive means (not shown). The stencil screen 10 includes a fine wire mesh cloth 14 and a suitable stencil coating 15 with openings 16 therethrough so as to define a desired pattern to be printed. The stencil screen may also be completely metal to obtain greater strength, toughness and conductivity. The stencil screen It} may also be attached to a set of timing belts or any other suitable accessory (not shown) which can be driven by rollers 11 and 12 synchronously with a conductive conveyor belt 18. An object to be printed upon, such as a paperboard substrate 19, may be registered with the stencil screen 10 by a plastic push rod 20, and the substrate 19 is moved with the conveyor belt 18 synchronously with the speed of the stencil screen 10.

A pair of tubes are provided with a plurality of corona pins 26, as is best shown in FIGURE 2, so that a mixture of air and toner particles can be blown out of the hole that surrounds each corona pin 26. The corona pins are preferably covered by plastic lids 27 in order to prevent ionic discharge and toner deposition where it is on the return portion of the loop of stencil screen 10. As is shown in FIGURE 1, the stencil screen 10 includes a return portion 28 and a printing portion 29. The substrate 19 is disposed between the printing portion 29 and the conveyor belt 18. Toner particles which are charged by the corona discharge, at the corona pins 26, are accelerated toward the printing portion 29 and pass through apertures 16 so as to be deposited upon the substrate 19 to form the desired printed pattern.

Air is provided by a blower and is blown into the tubes 25 through a toner container 36 which includes a supply of toner particles 37. The toner particles 37 are cloudized by application of a variable speed vibrator 38,

but a bypass of the air can also help by fluidization of the toner having air streaming into the toner through tube 93. Valve 94 can set a desired ratio between the direct and indirect airstreams. The cloudized toner particles are then picked up by the air stream, blown through the openings of tubes 25 and charged (preferably negative) by the corona pins 26 which are connected through a conductor 39 to a high voltage power supply 40. The voltage of power supply 45) is applied between the corona pins 26 and the conveyor belt 18 by means of sliding contact 41 and a conductor 42. Thus, a cloud of toner particles is charged and repelled through stencil screen 10 toward the conveyor belt 18, but is intercepted by the substrate 19 on which the image is to be printed.

The voltage of power supply is adjustable and is preferably between 10 and 20 kilovolts depending on the dimension of the air gap, the humidity of air, the amount of toner particles being used, and the speed of the substrate through the printing device.

Stencil screen 10 may also be kept on a variable potential by means of a sliding brush t5 which is connected by a conductor 46 to a variable power supply 47. The conductor 46 connects the negative side of the power source 47 to the brush and to the stencil screen 10, while a conductor 48 connects the positive side of the power source 47 to the conductor 42 and, hence, to the sliding contact 41. It is preferred that the power source 47 be capable of varying the voltage between O-5 kilovolts.

Toner particles which strike the stencil screen 10 may be prevented from clogging the stencil screen 10 by one or more cleaning brushes which are rotatably driven to dislodge toner particles from the stencil screen It). If additional cleaning is necessary, an air blast nozzle 56 can be applied on the return portion 28, of the stencil screen 10, and a vacuum channel 57 may be provided adjacent thereto so as to collect toner particles which are forced from the stencil screen 10 by the air blast nozzle 56. This helps to prevent clogging of the stencil screen and cleans the outside printing surface of the screen more thoroughly and, thus, produces a finer picture. If desired, the vacuum channel 57 may be eliminated and the air blast nozzle 56 may be used to increase the speed of toner particles as they are moved toward the printing area.

An auxiliary corona unit, generally indicated by the numeral 65, is preferably utilized to spray positive ions on the upper surface of the substrate 19 prior to the printing operation. Thus, the speed of the printing process can be increased even if substrates with lower conductivity such as dry paper are used and toner particles are held more securely to the substrate after being deposited thereon. The auxiliary corona unit includes a power source 66 having its positive side connected to corona pin 67 by a conductor 68, and the negative side of the power source 66 is connected by a conductor 69 to sliding contacts 70 disposed in electrical contact with the conveyor belt 18.

After the substrate 19 has been passed beneath the stencil screen it), toner particles deposited upon the substrate 19 may be fused thereto by exposure to a suitable heat source, such as an infra-red radiator, or by exposure to a suitable solvent for the toner particles.

An important feature of this invention is in the provision of means for controlling the darkness of the image deposited upon the substrate 19. The darkness of the image can be adjusted by the voltage of power supply 40 or by the voltage of power supply 47. However, this adjustment has to be fed back to control the amount of toner in the air stream by varying the speed of vibrator 38. Both processes can be made automatic by the electronic circuit which is shown in FIGURE 3. A photosensitive resistor element is shown as being connected in series with a variable resistor 76. An electron tube 77 has a grid 78 which is connected to the common point of photosensitive resistor element 75 and variable resistor 76. This circuit is fed by a transformer 79 which is connected through a suitable rectifier 80 and filtering means 31, 82 and 95.

The photosensitive resistor element 75 may, as is shown in FIGURE 1, be used to sense toner density inside the space confined by the loop of the stencil screen 10', or a photosensitive resistor element 75' may be used to sense the average darkness of a chosen strip of the printed image area on the moving substrate 19. Thus, the lowest limit of the darkness of the printed image can be defined by the characteristics of the electron tube 77 and by the manual presetting of the variable resistor or potentiometer 7 6 which predetermines a certain value for the photosensitive resistor 75 in order to produce the right voltage on the grid 78 of tube 77. When the grid 78 reaches that potential, a relay 35 starts a motor 86, as is shown in FIGURE 1, which (through a higher gear ratio) starts to turn a variac 87 of power supply 40 toward higher voltages. As a result, the printed image gets darker, the resistance of photosensitive resistor 75 or 75' gets higher,

the grid 78 gets less positive, and relay 85 kicks out. If this process should continue and the image gets darker than the desired degree, a relay 90 also would kick out. he relay 90, unlike relay 85, is a normally closed relay, and it is wired in a way that, when it is knocked out, it rotates the motor 86 of the variac 87 in the opposite direction. On the same shaft with this variac 87 there is a variable resistor which is the regulator of vibrator 38. Thus, the toner deposition and the amount of toner delivered can be coordinated.

A simplified solution is to use the photosensitive resistor 75 to control the vibrator directly and alone. in this case the voltages are preset and the two relays 85 and 90 govern the variac 92 directly, in very much the same manner as was described in connection with variac 87. This version is shown in FlG URE 2 where the relays 85 and 90 are connected to variac 92 which, in turn, is connected to the controlling resistor of vibrator 38.

While preferred forms and arrangement of parts have been shown in illustrating the invention, it is to be clearly understood that various changes in details and arrangement of parts may be made without departing from the spirit and scope of the invention, as defined in the appended claimed subject matter.

We claim:

11. Apparatus for cloudizing toner particles, said apparatus comprising a container for a supply of toner particles, blower means for creating a How of air through said container, vibrator means connected to said container for vibrating said container for agitating said supply of toner particles to form a cloud or" toner particles within said flow of air, sensing means for sensing toner density in said cloud, and means connecting said sensing means to said vibrator means for varying the vibration of said container inversely to an increase or decrease in the toner density.

2. In an electrostatic screen printing machine of the type utilizing toner particles for printing upon a substrate, an endless stencil disposed to form a loop includ ing a printing portion and a return portion, feed means for introducing toner particles to the interior of said loop and directed toward the printing portion of said loop, means for positioning said substrate adjacent to the said printing portion of said loop on the side opposite thereof from said feed means, and means for establishing an electric field for accelerating said toner particles toward the printing portion of said stencil screen, whereby some of said toner particles are stopped by said stencil screen and others of said particles pass through apertures in said stencil screen and impinge upon said substrate, cleaning means disposed adjacent to said stencil screen for dislodging toner particles from said stencil screen, said cleaning means comprising brush means disposed within said loop and in contact with said stencil screen, and air blast means and vacuum means disposed respectively on opposite sides of the return portion of said loop whereby toner particles dislodged by said brush means are blown from said stencil screen by said air blast means and collected by said vacuum means.

3. Apparatus for cloudizing toner particles, said apparatus comprising a container for a supply of toner particles, a tube having a plurality of holes therein, said tube being in fluid fiow communication with said container, a plurality of corona pins extending from said tube through said holes with the free end of said pins being exterior of said tube, and blower means in communication with said container for creating a flow of air through said container and said tube for carrying toner particles through said tube, through said holes and then past the free ends of said corona pins outwardly of said tube.

4. Apparatus as defined in claim 3 wherein said holes are only slightly larger than said corona pins whereby air-borne toner particles are precluded from becoming agglomerated.

5. Apparatus for cloudizing toner particles, said apparatus comprising a container for a supply of toner particles, an elongated tube having an open end and a closed end, said open end being in fluid flow communication with said container, a plurality of holes formed in said tube and spaced along the length of said tube, a plurality of corona pins extending through respective ones of said holes and having the free ends of said corona pins located exteriorly of said tube, and means for establishing a flow of fluid for carrying toner particles from said container, through said tube and holes, and past said free ends of said corona pins.

6. Apparatus as defined in claim 5 wherein said tube is formed of non-conductive material for precluding arcing from said free ends to said tube.

Reirerences Cited UNITED STATES PATENTS 1,730,195 10/1929 Davis 302-53 1,346,044 7/1920 Moore 15-308 2,219,208 10/1940 Knight 222193 2,536,902 1/1951 Bechett 15345 2,725,304 11/1955 Landrigan et al. 2,742,970 4/1956 Bauman et a1. 302-53 X 2,784,109 3/1957 Walkup. 2,800,432 7/1957 Weinrich 30252 X 2,824,813 2/1958 Fauser et al. 2,850,214 9/1958 Rooney 222193 2,859,129 11/1958 Hayford. 2,894,139 7/1959 Magroder et a1 250-495 2,956,487 10/1960 Giaimo. 2,965,481 12/1960 Gundlach. 2,996,575 8/1961 Sims. 3,003,176 10/1961 Goyette 15-308 X 3,017,339 1/1962 Dewey 204-465 3,081,698 3/1963 Childress et al. 101114 X 3,134,513 5/1964 Ashman 222-193 3,218,967 11/1965 Childress. 3,295,440 1/1967 Rarey et al. 101114 FOREIGN PATENTS 81,920 9/ 1956 Denmark.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Examiner,

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