Method Of Reducing Internal Matrix Arcing In Electrostatic Printing Tubes

Abstract

Internal matrix arcing is reduced by providing the tube with an internal electrically conductive coating which is anhydrous or by applying hydrated material and baking it in air to dehydrate the material before the matrix is applied to the tube.

Parent Case Text

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a division of Ser. No. 690,688, filed Dec. 14, 1967, which in turn is a streamlined continuation of Ser. No. 483,510, filed Apr. 30, 1965.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrostatic writing tubes and more particularly to improvements therein and in the method of manufacturing the same.

Electrostatic writing tubes comprise an hermetically sealed, electrically insulating, substantially funnel-shaped envelope containing an electron gun for emitting a beam of electrons and a matrix spaced therefrom on which the electron beam impinges. The matrix is comprised of a plurality of laterally arrayed, insulatingly spaced apart conductive members which extend from the internal surface of the matrix to the external surface thereof. An internal anode in the form of an electrically conductive coating of some suitable material, such as Aquadag, is provided within the tube and extends from the electron gun to a position spaced from the matrix.

In the apparatus with which the tube is utilized is a conductive plane and the matrix of the tube is aligned with and spaced from this plane. Since this conductive plane is maintained at anode potential it is often referred to as an external anode. Also included in this apparatus are means for causing the electron beam to scan the matrix in a given manner and means for passing a dielectric material, such as paper, between the tube and the conductive plane, or external anode.

In operation the electron beam emanating from the electron gun is turned on or off in accordance with electrical signals fed to the cathode of the gun from associated equipment and the beam is caused to scan the matrix in a given manner in accordance with other signals. As the electron beam strikes one of the conductive members on its internally disposed surface the member will begin to charge toward cathode potential. When the member has substantially reached cathode potential it will discharge to the external anode. If a dielectric material, such as paper, is present between the matrix and external anode the discharge will pass to the paper leaving an electrostatic charge substantially of the same diameter as the conductive member thereon.

By causing the paper to move transversely past the tube in the space between the tube and the external anode and by selectively applying the electron beam to the interior surface of the conductive members, a selected code array of electrostatic charges, which may be in the form of letters or numbers, will be deposited on the paper. The charges, which have a given polarity, may be subsequently developed by dusting the paper with a dielectric powder whose grains carry signs of the opposite polarity. The powder will cling to the charged areas as distinguished from the uncharged areas and may then be permanently fixed to the paper by heat, pressure, or other suitable means.

Tubes of the type described are extremely useful in many applications such as printing labels or the like. When fed the proper information words or names and addresses may be printed at tremendous speed and of size smaller than the average elite or pica typewriter type. One particular application of a tube of this type is in printing labels containing thereon the names and addresses of magazine subscribers. Since these labels will be subsequently attached to the magazine and used as the mailing label, it will be seen that to avoid errors in delivery the label must be clearly printed. For example, if the letter "P" were improperly printed it might appear as the letter "F" and could conceivably cause delivery of a magazine or other article to the wrong address. A more detailed account of the operation of a printing system of this type may be found in U.S. Pat. No. 2,894,799.

One of the first methods employed in assembling these tubes comprised coating the internal surfaces of the envelope with Aquadag, care being used to see that no Aquadag was applied to the matrix area. The envelope was then baked in air to drive off occluded water vapor and other gases. The insertion of the gun and its attached stem was accomplished next and then the stem was sealed to the neck of the tube and the tube was exhausted and tipped off.

Tubes built in accordance with this method frequently produced improper printing because of a condition known as "butterflies." Butterflies are caused by arcing on the internal surface of the matrix. These arcs may discharge from conductive member to conductive member, from member to glass or from glass to glass and, since they are deposited on the dielectric material as unwanted electrostatic charges, they appear on the printed matter (after developing) as a combination of open spaces and/or extra dark areas or simply as a spot where no spot should be.

While it is virtually certain that the "butterflies" are caused by internal matrix arcing, the exact cause of the arcing has never been determined beyond all doubt.

One current theory, which subsequent discussion will shown appears to be reasonable, holds that some form of physical or chemical change occurs on the internal surface of the matrix either during processing or operation of the tube and that these changes are responsible for the arcing.

A prior method of combating arcing involved sandblasting the internal surface of the matrix during processing with a fine grain sand. This roughened the internal surface of the matrix and virtually eliminated internal matrix arcing and associated butterflies; however, it introduced another problem in the form of insufficient back-bar efficiency and uneven current distribution across the matrix. Back-bar efficiency is the ratio of available output current (at the external surface of the matrix) to total anode input current, expressed as a percentage of total anode current. The term is derived from the method of determining the efficiency which consists of holding to the external surface of the matrix, during operation, a "back-bar," a bar of insulating material having a conductive surface and which is connected, through a meter, to anode potential.

To be acceptable a tube should possess a back-bar efficiency of at least 60 percent of total anode current and higher ratings would definitely be desirable. Tubes having a sandblasted matrix can be processed to provide this required minimum efficiency and a substantially uniform current distribution by "burning in" the matrix after completion of the tube. That is, operating the tube with a full scan for a definite period of time. This approach, while workable, is a time consuming and costly operation.

Another approach utilized to return the back-bar efficiency rating to acceptable levels is to acid etch the matrix after sandblasting with a mixture of hydrofluoric acid and sulphuric acid in the ratio of 1 to 1. The acid etch smoothed out somewhat the roughened surface of the matrix and improved the back-bar efficiency, apparently by reducing the amount of secondary emission from the internal matrix surface. Tubes produced by this method had reduced internal matrix arcing and acceptable back-bar efficiency but, again, another problem was introduced; i.e., the tubes did not have an acceptable life expectancy. The reason for this may be made clear by an understanding of the matrix construction.

A current type of writing tube now in use has a matrix area approximately 3 inches long by 3/8 inches wide. It is composed of a plurality of electrically conductive members, such as wires, having a diameter of 0.0005 inches spaced apart on a center-to-center distance of 0.003 inches. Since these members are all insulated one from another by glass, a matrix of the size noted above would contain over 40.000 individual glass to metal seals. It is felt that the sandblasting and acid etching procedures used heretofore had a deleterious effect on these seals since an undesirable number of tubes produced by this method would develop air leaks in the matrix area shortly after they were put into use.

OBJECTS AND SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to enhance the manufacturing techniques utilized in the construction of electrostatic writing tubes.

It is another object of the invention to produce an improved tube of this type.

Still another object of the invention is the reduction of time utilized in the manufacture of these tubes.

Other objects are the provision of an electrostatic writing tube characterized by substantial freedom from internal matrix arcing, improved resolution, improved back-bar efficiency, uniform current distribution across the matrix, and long life.

These objects are achieved in one aspect of the invention by providing a method of manufacturing electrostatic writing tubes which comprises providing selected interior surfaces of the tube with a substantially anhydrous, electrically conductive material. This aspect of this method eliminates five of the steps previously utilized in construction with consequent cost savings and results in a tube having the desirable characteristics of substantial freedom from internal matrix arcing, improved resolution, improved back-bar efficiency, uniform current distribution, and long life expectancy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a prior art method utilized in the manufacture of electrostatic writing tubes;

FIG. 2 is a flow diagram of an embodiment of the method of this invention; and

FIG. 3 is a longitudinal sectional view of an electrostatic writing tube constructed in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the disclosure and appended claims in connection with the above-described drawings.

In the course of experimentation into methods of improving electrostatic writing tubes it has been discovered that by applying an anhydrous, electrically conductive material to selected interior surfaces of the tube, in place of the Aquadag and with the consequent elimination of the preliminary air bake to drive off occluded water vapor, a tube is produced that has virtually no internal matrix arcing. Apparently, a reaction occurred between gases and water vapor driven off in the air bake and the internal surface of the matrix. As noted above with reference to the prior art and as is shown in FIG. 1, the internal matrix arcing was reduced by sandblasting even though the air bake was performed subsequent thereto. Therefore, it may be assumed that roughening the surface prevented or greatly retarded whatever this reaction was. The exact nature of the deleterious phenomena that occurred are unknown at this time.

Further advantages in this method occur because of the elimination of the sandblasting or roughening of the internal matrix surface. Since roughening of the surface is known to cause an increase in secondary emission and, since as noted hereinbefore, this apparently caused a decrease in back-bar efficiency, it became possible to provide a polished internal matrix surface. By providing a surface having a smoothness comparable to that obtainable with a 400 grit polish, back-bar efficiencies ranging from 65 percent to 75 percent of total anode current are achieved. These are well into the acceptable range and, of course, eliminates the need for any pre-use "burn-in."

Utilization of this method also provides for increased resolution. The improved resolution occurs because the greater back-bar efficiency of tubes produced by this method allows reducing the total anode current while still maintaining a current output value equivalent to that of tubes having lower efficiency. Since the beam spot diameter is dependent upon beam current, i.e., total anode current, (the higher the beam current, the larger the spot diameter) a lower beam current provides a smaller beam spot diameter with subsequent better resolution.

A still further improvement will be explained with reference to FIG. 3 wherein is shown an electrostatic writing tube 10 comprised of an envelop 12 having a faceplate 14 closing one end thereof. The faceplate 14 contains therein a matrix 16. The envelope 12 and faceplate 14 are preferably of some suitable electrically insulating material, such as glass; however, the faceplate may be metal provided that the matrix is insulated therefrom. Spaced from the matrix and aligned therewith is an electron gun 18 for providing a beam of electrons 20. An internal, electrically conductive anode coating 22 is disposed about the inside surface of the envelope and extends from a position spaced from the matrix to the electron gun. The anode electrode of gun 18 is electrically connected to anode coating 22 by suitable means, as snubbers 24. An anode connector 26 contacts the coating 22 and projects from the envelope so that a suitable source of potential may be connected thereto. Adjacent the outside surface of the matrix 16 and spaced therefrom is an external anode electrode 28 which is connected to the same potential source as coating 22.

A dielectric material 30 such as paper is passed between external anode 30 and matrix 16 for the reception of electrostatic charges.

In tubes constructed by prior methods; i.e., with the Aquadag coating, it was customary to leave at least one small uncoated area inside the tube adjacent the matrix. This area is indicated in the drawing as 32. Since the Aquadag is opaque, this area was necessary to permit visual inspection of the beam alignment. Problems again were present however, in that lack of a conductive coating in this area caused capacitive charges to build up thereon and cause misregistry of the beam when it was in that region.

This problem may be avoided with this invention by the use of a transparent conductive coating such as stannous chloride, which is anhydrous and does not need an air bake. Another applicable approach is to coat only the viewing area 32 with a transparent conductor and the rest of the tube with an opaque, anhydrous conductor, such as aluminum. Obviously, suitable masking techniques would be necessary.

When stannous chloride is used as the internal conductive coating it is customary to apply a small amount of Aquadag around the internal end of anode connector 26 and also around the neck area contacted by the snubbers. This ensures good electrical contact at these points because of the greater electrical conductivity of the Aquadag relative to the stannous chloride. The small amount of moisture added to the tube with the addition of the Aquadag is adequately driven off during exhaust and no separate bake is required.

It is to be noted that in addition to applying an anhydrous coating to the tube it is possible to apply a hydrated or hygroscopic coating to the tube before the matrix is attached thereto. In this instance the coating and tube would be air baked before the attachment of the matrix and then would be maintained in an anhydrous atmosphere until such attachment could take place. Thus, by this technique a hydrated coating would be applied to the envelope, but an anhydrous coating would be provided for the tube.

The invention described above has provided significant advances in the art of manufacturing electrostatic writing tubes. The elimination of five previously performed steps has resulted in considerable cost savings. Additionally, the tube produced has substantially no internal matrix arcing, improved resolution, improved back-bar efficiency, uniform current distribution and greater life expectancy than tubes produced by methods used heretofore.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

I claim:

1. A method of reducing internal matrix arcing in electrostatic writing tubes having a faceplate matrix portion, an envelope, and an electron gun, which includes the steps of: coating said tubes with an anhydrous material to form an electrically conductive coating on the interior of said envelope to eliminate the possibility of a hydrated vapor contacting the interior surface of said matrix whereby the prevention of a deleterious reaction between said vapor and said matrix is achieved.

2. A method of reducing internal matrix arcing in electrostatic writing tubes having an envelope and an electron gun which includes the steps of: coating said tubes with a hydrated material to form an electrically conductive coating on the interior of said envelope, baking said hydrated material in air to dehydrate said hydrated material and to eliminate the possibility of a hydrated vapor contacting the interior surface of said matrix whereby the prevention of a deleterious reaction between said vapor and said matrix is achieved, and subsequently sealing a matrix to said envelop.