Method of operating a drop generator that includes the step of pre-pressurizing the liquid manifold

Abstract

Method of operating a drop generator while avoiding spattering during start up and termination of operations. The method includes pressurizing the drop generator manifold with air or other gas and with a flushing liquid formulated to leave substantially no residue upon evaporation, before pumping coating material to the manifold. In stopping the drop generator the flow of coating material to the manifold is replaced with a flow of flushing liquid and the flow of flushing liquid is then terminated and, simultaneously, an evacuation line leading to a low pressure source is opened.

Description

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 3,560,641, 3,586,907 and 3,661,304 are directed to noncontacting coating systems wherein a liquid coating material, such as ink, is pumped under pressure to a manifold communicating with a series of small diameter orifices. As the coating material is ejected through the orifices under pressure, it forms fine filaments of coating material which break down into series of discrete drops. At the point where the drops break from the filaments they pass through charging rings which, depending upon the pattern of coating material desired on a receiving member conveyed beneath the drop generator, either charge or do not charge each individual drop of coating material.

An electrostatic deflecting field is set up downstream of the charge rings and all drops which receive a charge while passing through the charge rings are deflected from their trajectory by the deflecting field. A catcher is also associated with the system to catch those drops which it is desired to prevent from reaching the receiving member. In this way it will be seen, a patterned coating, such as printing, is applied to the receiving member.

In the operation of a drop generator of this type, it will be apparent that it takes some discrete pressure, hereinafter termed the operating pressure, to produce a filament of sufficient velocity to overcome forces, such as surface tension forces, tending to retard flow of the coating material through the orifices.

If the flow of coating material to the drop generator is commenced by merely opening a supply line to the manifold, it will be apparent that the pressure build up in the drop generator from zero to the operating pressure will occur over some finite time period.

During this period, when the pressure acting on the coating material has not yet reached operating pressure, a free jet will not be produced, but instead, a pendulous mass of coating material will collect at each orifice which weeps liquid coating material therefrom. As the pressure acting on the coating material increases a jet will eventually be produced inside the mass of liquid and finally break from the mass in an uncontrolled manner, only stabilizing after the excess liquid at the orifice has been drawn away by entrainment in the jet.

Obviously this will result, not only in a more lengthy start up procedure, but also in spattering of the coating and the collection of coating material on the components of the generator. Since the coating material is electrically conductive this can result in shorting of the various electrical components, such as the charge rings and deflecting field electrodes. Additionally, the evaporation of the coating material will leave a residue on the components of the drop generator which will eventually affect its operation.

It will also be apparent that if drop generation is terminated by merely terminating the supply of coating material, the pressure in the manifold will decrease over a finite time period, again causing masses of coating material to collect at the orifice.

SUMMARY OF THE INVENTION

In method and apparatus in accordance with the present invention the coating material does not contact the filament forming orifices until the pressure necessary to form a free filament of coating material has been reached in the manifold. This is accomplished by first pressurizing the manifold with a gas, such as air, to a pressure at least equal to or preferably substantially above the operating pressure required for production of a free standing filament of coating material.

Thereafter, the coating material is pumped to the manifold at or above operating pressure and, since the manifold has already been pressurized before introducing the coating material, the coating material arrives at each of the orifices at or above operating pressure and immediately forms a free standing filament issuing from the orifice and forming a series of discrete drops.

By this method the collection of pendulous masses of coating material at the orifice and the contamination of the generator components is substantially avoided. There may, however, be a fine spray associated with the transition from gas to liquid coating material at each of the orifices as the coating is ejected therefrom. If this momentary spray or misting is found objectionable, it may also be substantially eliminated by interposing an additional step before the pumping of the coating material to the manifold.

Thus, a flushing liquid characterized by a substantial absence of residue upon evaporation, may be pumped to the manifold before the coating material is pumped thereto. Thus, if any misting or spraying occurs during the transition from gas to liquid at the orifices the liquid will quickly evaporate, with negligible residue resulting. After free standing filaments of flushing liquid are established at each of the orifices the supply of flushing liquid to the manifold is terminated and simultaneously replaced with a flow of coating material at or above operating pressure. Also, at this point any gas which remains entrained in the manifold may be bled therefrom.

In terminating operation of the drop generator the supply of coating material is terminated and simultaneously replaced with a supply of the flushing liquid. Thereafter, the supply of flushing liquid is terminated and simultaneously a line is opened from the manifold to a waste sump operating at a pressure substantially below atmospheric. This sudden reduction of pressure in the manifold is sufficient to terminate the jets of flushing liquid without producing masses of liquid at the orifices or the formation of erratic drops.

Air may conveniently be used as the pressurizing gas and the flushing liquid may be any liquid which evaporates without appreciable deposit of residue. In this regard, a mixture of approximately 50 percent distilled, deionized water and 50 percent denatured alcohol appears satisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic showing of a drop generator in accordance with the present invention;

FIG. 2 is a cross sectional view taken on line 2--2 of FIG. 1;

FIG. 3 is an enlarged cross sectional view showing the formation of coating material accumulations that result when pressure is allowed to build up gradually at the orifices; and

FIG. 4 is an enlarged cross sectional view similar to FIG. 3 but showing the formation of filaments and drops of coating material in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIGS. 1 and 2 of the drawings, a drop generator 10 in accordance with the present invention may include manifold 12 having a chamber 14 formed therein. Mounted beneath the manifold 12 is an orifice plate 16, a spacer plate 18, a charge ring plate 20, a pair of deflecting electrodes 22 attached to the charge ring plate, as at 23, and a catcher 24 spaced from the electrodes by mounting means 26.

Coating material supplied to the chamber 14 will be ejected through the orifices 28 to form fine filaments which break up into discrete drops of coating material. It is desirable that if a charge is to be applied to a particular drop it is applied at approximately the point at which the drops break from the filaments. Thus, the spacer plate 18, having a series of openings 30 formed therethrough, spaces the charge ring plate 20 at the proper distance from the orifice plate 16 such that the charge rings 32 charge each of the drops of coating material just as they break from their respective filaments of coating material.

Thereafter, the electrodes 22 deflect all charged drops toward the blade 34 of the catcher 24 while uncharged drops are allowed to impinge on a receiving member 36 conveyed in any convenient manner past the drop generator, as indicated by the arrow in FIG. 2 of the drawings. The above description is merely for purposes of background and for a more detailed description reference may be had to the two above noted U.S. Pat. Nos. 3,560,641 and 3,586,907.

With regard to the present invention, it will be seen that if the supply of coating material to the chamber 14 is commenced by merely opening a valve from a source of coating material, it will take some finite time interval until the pressure in the chamber 14 has built up to operating pressure, that is the pressure at which the coating material will overcome forces, such as surface tension forces, tending to prevent its being ejected from the orifices as free standing jets.

Thus, until pressure builds up to operating pressure the coating material will tend to form pendulous masses, as indicated at 38 in FIG. 3 of the drawings, which weep coating material downwardly, contaminating other components of the generator, such as the charge rings 32. To avoid this, chamber 14 is prepressurized with the system shown somewhat schematically in FIG. 1 of the drawings.

Thus, a gas port 40 is provided leading to one end of the chamber 14 while a liquid port 52 communicates with the opposite end of the chamber. A gas, such as air, is pumped through line 44 and valve 46 by means indicated at 48. A second line 50 also communicates with the gas port 40 and is provided with a valve 52 and a source of negative pressure as indicated at 54. A bleed line 55 also branches from port 40, controlled by valve 56.

Liquid port 42 may be fed by line 57, which includes a valve 58 and is connected with means, as indicated at 60, for pumping a flushing liquid through the line 57. Also connected to port 42 is a line 62, controlled by valve 64, and through which coating material may be pumped by pump 66.

With the above system the sequence of operation is as follows. All lines leading to the chamber 14 are closed. Next, valve 46 is opened, allowing air or other pressurizing gas to be pumped through line 44 by means of pump 48 at a pressure in excess of operating pressure. The pressure and flow rate of gas is such that the pressure in the chamber 14 quickly rises to a pressure above normal operating pressure, despite the escape of gas through the orifices 28.

Coating material may then be introduced into the system by opening valve 64 and pumping the coating at or above operating pressure through liquid port 42. The coating material will advance through the chamber 14 in a coherent wave, from right to left as seen in FIG. 1 of the drawings. Since it is at or above operating pressure as it reaches each of the orifices, free standing jets are formed, as seen at 70 in FIG. 4 of the drawings, which break down into series of discrete drops 72. Preferably, valve 46 is closed about 0.3 seconds after the opening of valve 64.

As the flow at each of the orifices transfers from gas to liquid, a slight misting may be experienced. If this is found to be objectionable, a flushing liquid may be introduced into the manifold before the introduction of coating material. Thus, the above procedure may be modified as follows.

With all valves initially closed, valves 46 and 58 are opened, allowing gas to be pumped from source 48 and a flushing liquid to be pumped from source 60, both the gas and flushing liquid being at a pressure in excess of operating pressure. Again, the liquid entering the chamber 14 will advance as a coherent wave, replacing the gas flow at each of the orifices 28 with a liquid flow at or above operating pressure.

While the transfer from gas to flushing liquid may also result in a fine mist or spray at each of the orifices 28, this will not affect or contaminate the components of the generator since the flushing liquid is formulated such that it leaves virtually no residue upon evaporation.

After the free standing jets of flushing liquid are formed at each of the orifices 28 and the valve 46 has been closed, valve 64 is opened and simultaneously valve 58 is closed. This allows coating material at or above operating pressure to flow through line 62 and liquid port 42 into the chamber 14, displacing any flushing liquid therein with coating material. Thereafter, any air or other gas which may remain in the system may be bled out through line 55 and valve 56.

When it is desired to terminate operation of the drop generator, valve 64 is closed and valve 58 opened, causing the flow of coating material to be replaced by a flow of flushing liquid. Thereafter, valve 58 is closed and valve 52 is simultaneously opened, causing both the flow of flushing liquid to terminate and the evacuation of the system to a waste sump operating at a pressure substantially below atmospheric pressure. This sudden reduction in pressure in the system is sufficient to terminate the jets 70 without producing erratic drops below the orifices or allowing coating material to collect in pendulous masses about the orifices.

Under normal operating conditions, providing pressurizing gas at approximately 18 psi, flushing liquid at approximately 20 psi and coating at approximately 11 psi has been found satisfactory for the usual range of viscosities found in coating materials such as ink and with an orifice plate having 600 orifices of 1.5 mil diameter. Additionally, evacuating the drop generator upon termination of operations to a waste sump at approximately 15 inches of mercury below atmospheric pressure has been found satisfactory.

From the above it will be apparent that the present invention provides method and apparatus for operating a drop generator without the problems normally associated with start up and shut down procedures.

While the methods and forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made therein without departing from the scope of the invention.

Claims

What is claimed is:

1. A method of commencing operation of a drop generator which includes orifice defining means and a manifold communicating with said orifice defining means for providing a supply of liquid coating material thereto comprising:

a. pressurizing said manifold with a gas to a pressure above the coating pressure necessary to cause said coating material to be ejected from said orifice defining means as a free standing filament which forms a series of discrete drops, and

b. thereafter introducing said coating material into said manifold at a pressure no less than said coating pressure.

2. The method of claim 1 wherein said step of pressurizing said manifold with gas comprises:

a. pressurizing said manifold with gas at approximately 18 psi.

3. The method of claim 1 wherein said step of introducing coating material into said manifold comprises:

a. introducing said coating material into said manifold at approximately 11 psi.

4. The method of claim 1 further comprising:

a. adding a flushing liquid to said manifold after said step of pressurizing said manifold with gas and before introducing said coating material into said manifold.

5. The method of claim 4 wherein said step of adding flushing liquid comprises:

a. adding said flushing liquid at approximately 20 psi.

6. The method of claim 4 further comprising:

a. discontinuing said adding of said flushing liquid to said manifold while continuing said step of introducing said coating material into said manifold,

b. discontinuing introducing said coating material into said manifold and commencing adding flushing liquid to said manifold, and

c. evacuating said manifold.

7. The method of claim 6 wherein said step of evacuating said manifold comprises:

a. evacuating said manifold at a negative pressure of approximately 15 inches of mercury below atmospheric pressure.

8. The method of claim 1 wherein said step of pressurizing said manifold with a gas comprises:

a. pumping gas under pressure into said manifold.

9. The method of claim 8 further comprising:

a. discontinuing said pumping of said gas into said manifold after said coating material is introduced into said manifold at said coating pressure.

10. A method of operating a drop generator which includes an orifice plate having means defining a plurality of small diameter orifices therethrough, a manifold communicating with said orifices for providing a supply of coating material thereto, and a gas port and a liquid port communicating with said manifold comprising:

a. pumping air into said manifold through said gas port at approximately 18 psi,

b. pumping a mixture of distilled, deionized water and denatured alcohol through said liquid port and into said manifold at approximately 20 psi,

c. discontinuing said pumping of air into said manifold,

d. discontinuing said pumping of said mixture after said manifold is full of said mixture and commencing pumping of coating material through said liquid port into said manifold at approximately 11 psi,

e. bleeding any remaining air in said manifold out through said gas port,

f. continuing pumping of said coating material into said manifold to cause fine filaments of said coating material to be ejected through said orifices and form a series of discrete drops,

g. discontinuing pumping of said coating material and commencing pumping of said mixture through said liquid port into said manifold, and

h. substantially simultaneously discontinuing pumping of said mixture and evacuating said manifold through said gas port at a negative pressure of approximately 15 inches of mercury below atmospheric pressure.