US3454399A

US3454399A - Apparatus and method for producing curved electrostatic printing screens

Info

Publication number: US3454399A
Application number: US473013A
Authority: US
Inventors: James W Edwards
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1965-07-19
Filing date: 1965-07-19
Publication date: 1969-07-08
Anticipated expiration: 1986-07-08
Also published as: DE1571853A1; GB1157688A; US3665851A; US3523502A

Description

CURVED July 8, 1969 J. w'. EDWARDS APPARATUS AND METHOD FOR PRODUCING ELECTROSTATIC PRINTING SCREENS Filed July 19, 1965 INVENTOR JAMES W. EDWARDS ATTORNEY APPARATUS AND METHOD FOR PRODUCING CURVED ELECTROSTATIC PRINTING SCREENSv Sheet of 5 Filed July 19, 1965 A f I INVENTOR 9 JAMES w. EDWARDS ATTORNEY U.S. Cl. 9636.4 2 Claims ABSTRACT OF THE DISCLOSURE A method for making stencil screens used in electrostatic stencil printing systems where the screens have masked and open areas enabling electroscopic ink to pass through the opened areas of the screen to form a desired image pattern on a substrate. The method of making the screens includes the steps of applying a photosensitive emulsion to a metal fabric, exposing the emulsion through an image pattern by means of light and washing the nonexposed portion of the sensitive emulsion. Thereafter, the screen is bent to produce a desired cross-sectional shape. The interstices of the non-image areas of the screen are filled with a metal powder which is then pressurized to cause the powder to become impacted in the fabric interstices. The fabric is thereafter heated to cause the metal to become sintered to create a substantially rigid member. A modification is disclosed in which a non-metal fabric is first metalized and then treated as above. In addition, a pressurizing apparatus in the form of a plug-like element accommodates an article inserted thereon. Pressure is applied from the interior of the pluglike element and the exterior atmosphere surrounding the plug-like element so that pressure can be applied op posite the surfaces of the article.

This invention relates in general to certain new and useful improvements in the method of making electrostatic printing equipment, and more particularly to an improved method and apparatus for making electrostatic printing screens having curvilinear shapes and which are used in electrostatic screen process printing.

The presently known techniques in electrostatic printing are described in U.S. Letters Patent No. 3,081,698 which relates to a method of electrostatic printing by elimination of pressure or contact between the printing element and the subject material being printed. This technique involves the transfer of a resinous based ink through an electrostatic field to an image-receiving media. The ink or pigments are usually in the form of a fine powder having a particle size which is small enough to pass through the interstices of the open areas of a stencil or so-called screen. A roller or similar mechanical device normally carries the ink particles to a point in close proximity to the stencil and where the ink is carried through the stencil by the electrostatic field to the image-receiving media. When the voltage is applied to the roller or element carrying the pigment, the particles acquire a charge. The charge is, of course, opposite to the backing plate and the ink particles are, therefore, accelerated through the openings or interstices in the open areas of the screen and toward the image-receiving media. The image-receiving media may consist of a mandrel which serves as a counter-electrode and which is capable of retaining the article to be printed. Thereafter, the ink will collide with and adhere to the article which is to be printed and the image is subsequently fixed by heat or solvent or a vapor or by other suitable means which are known in the prior art.

Since the initial development of the theory of electro- United States Patent 'ice static printing, there have been many attempts to print by electrostatic process principles on items having curvilinear shapes. Most of these attempts proved to be unsuccessful, for a number of reasons. In all of the electrostatic screen process printing apparatus thus far developed, the apparatus has necessitated the transfer of ink across a definite and appreciable space. However with curved articles, all portions of the screen were not equidistantly spaced from the surface of the article to be printed. As a result thereof, the printing was weak in some areas and extra heavy in other areas. Furthermore, because of this variable spacing between the screen and the substrate to be printed, the problems of image distortion effects arose.

It has recently been discovered that it is possible to print curvilinearly shaped articles using electrostatic screen process printing techniques by the employment of a curved electrostatic printing screen. It was recognized that it is necessary to employ a screen having substantially the same size, shape and contour of the article being printed, in the areas where the article is to receive the print. Moreover, it has been found that very good results have been obtained when the article to be printed is completely surrounded by an electrostatic printing screen and where all portions of the ink receiving surface of the article are equidistantly spaced from all points on the screen. However, to date, there has been no known and effective method of producing an electrostatic printing screen having a curvilinear shape which can be employed in electrostatic screen process printing techniques. Furthermore, there is no known method of producing a self-supporting screen which is capable of surrounding the article to be printed and yet being spaced sufiiciently to maintain an electrostatic field therebetween. Accordingly, it was necessary to produce screens with external tensioning devices which were massive and awkward to handle. Furthermore, it was difficult to adapt ink feeding mechanisms to these curved screens external members. The printing screens of this type were quite costly to manufacture and moreover, were not constructed with the desired degree of tolerance usually required in electrostatic screen process printing techniques.

It is, therefore, the primary object of the present invention to provide a method of making electrostatic printing screens for use in electrostatic screen process printing techniques.

It is another object of the present invention to provide a method of the type stated for producing electrostatic screens having self-supporting curvilinear shapes.

It is a further object of the present invention to provide a method for producing electrostatic printing screens of the type stated which requires a minimum amount of manual attention and thereby permits construction of a low cost electrostatic printing screen.

It is an additional object of the present invention to provide a method for producing screens of the type stated where the screens are characterized by simplicity, dependability, ruggedness and low cost.

It is also an object of the present invention to provide an apparatus for making curved electrostatic printing screens to be used in electrostatic screen process printing techniques.

It is another salient object of the present invention to provide an apparatus of the type stated which is relatively economical to manufacture and requires a minimum of manual attention in its operation.

With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement and combination of parts presently described and pointed out.

In the accompanying drawings (3 sheets):

FIGURE 1 is a perspective view of a wire mesh screen coated with a photosensitive emulsion and with a desired image projected thereon by a source of light through a negative having the desired image pattern;

FIGURE 2 is a perspective view of a hardening bath showing the wire mesh screen of FIGURE 1 submerged therein;

FIGURE 3 is a perspective view of the wire mesh screen formed into a truncated conically shaped section which is substantially similar in surface contour to the articles to be printed;

FIGURE 4 is a vertical sectional view, partially broken away, of a plug or die frame which is constructed in accordance with the present invention and supports the conically shaped wire mesh screen in a liquid-metal bath;

FIGURE 5 is a vertical sectional view, partially broken away, showing a unique type of cooperating die frame constructed in accordance with and embodying the present invention for producing the conically shaped electrostatic printing screen of the present invention;

FIGURE 6 is a fragmentary sectional view taken along line 66 of FIGURE 5;

FIGURE 7 is a side elevational view, partially broken away, showing the conically shaped electrostatic printing screen sintered in a furnace;

FIGURE 8 is an exploded perspective view showing the method of attaching end supporting rings to the conically shaped electrostatic printing screen;

FIGURE 9 is a perspective view of the final electro static printing screen; and

FIGURE 10 is a fragmentary sectional view taken along line 10- 10 of FIGURE 9.

Generally speaking, the present invention provides a method and apparatus for producing curved screens to be used in electrostatic printing operations. Generally, the curved screens are of the type described in copending application Ser. No. 472,982, filed July 19, 1965, now US. Patent No. 3,302,561, which relates to electrostatic screen process printing on curvilinearly-shaped articles. In the process of the present invention a direct screen photosensitive emulsion is applied to a wire mesh which is preferably held in a screen chase. The screen is then exposed to light through a photographic negative of the required print or design which is ultimately to be produced on a substrate. The wire mesh is thereafter washed in hardening bath which is designed to harden the exposed portion of the photosensitive emulsion and rinse away the non-exposed portion of the emulsion, thereby leaving a positive image on the screen. In other words, the image areas will be filled with the exposed photosensitive emulsion since this area was exposed to the light. Thereafter, the metal may be cut as desired and formed into a desired shape, such as a truncated-conically-shaped section, which will surround and generally conform to the overall shape of a container which is ultimately to be printed. The end margins of the section may be tacked or spot welded in order to form the truncated section. Thereafter, the comically-shaped screen is supported on a suitable pluglike mandrel-shaped die which is especially designed for the purpose of the present invention. The screen and mandrel-die is then dipped into a suspension or container of finely powdered metal where a desired metal or alloy is filled in the interstices of the open mesh portions of the screen. Thereafter, the mandrel-die supporting the comically-shaped screen is withdrawn from the metal suspension and permitted to sufficiently dry, where the metal particles remain in the interstices of the open mesh portions of the screen.

The conically-shaped screen is then fitted between the mandrel-die which has an outer rubber surface and a second die consisting of a fairly hard rubber sleeve which surrounds the screen. The mandrel-die, sleeve and screen are all secured by a band at their upper ends thereby providing a pneumatic seal between the first and second comically-shaped rubber sections which retain the screen. The mandrel-die is provided with suitable fluid apertures permitting communication from the mandrel to the inner rubber mold. The apparatus is then submerged in a fluid pressure tank and the pressure is increased so that the metal in the interstices of the screen is impacted by the pressure on both surfaces of the comically-shaped rubber dies.

After the metal has been sufficiently impacted in the surfaces of the screen, the screen is then placed in a suitable furnace for heating the screen at least to the annealing temperature of the metal impacted in the interstices so that the metal is sintered into the screen. After the metal has been hardened, the screen has a fairly rigid shape and may be subjected to a final shaping, if desired. End rings are then placed on the opposite ends of the screen to provide further rigidity.

Referring now in more detail and by reference characters to the drawings which illustrate a preferred embodiment of the present invention, the process of the invention and the electrostatic screen produced thereby are more fully illustrated in detail. In FIGURE 1, a metal fabric or so-called wire mesh 1 is illustrated. The metal fabric may be of the plain weave or the twill weave. However, in either weaves the openings are square. For the purposes of electrostatic screen process printing, the metal fabric should be formed of a No. No. 350 wire mesh. In unusual cases, coarser cloths than No. 80 may be employed and similarly in unusual cases, finer cloths than No. 350 may be employed. The wire mesh number is the standard employed in the metal fabric industry Where for example No. 80 square mesh cloth would have 80x80 openings per square inch or 6,400 openings per square inch while No. 300 metal cloth would have 90,000 openings per square inch. The wire mesh 1 may be formed of any suitable metal which has the desired gauge size and which is electrically conductive, at least for purposes of electrostatic printing. An electrostatic field is to be maintained between a screen and a counter-electrode. Such suitable metals to be used are nickel, aluminum, stainless steel, copper, etc. Similarly, suitable alloys of metals may be employed such as a copper-zinc allow, or a copper-aluminum alloy, etc.

Thereafter, a suitable direct screen emulsion or sensitized photographic coating is applied directly to the surface of the metal fabric 1. Sensitized photographic coatings such as polyvinyl alcohol, polyvinyl acetate, modified polyvinyl plastics, commercial coatings, gelatinous coatings, transfer-type sensitized films and knife-cut fil ms may be deposited on the metal screen fabrics. The important step before the application of the sensitized coating is to insure that the metal fabric is cleaned perfectly. Since the metal is non-absorbent, it is essential that the fabric be clean and devoid of any oils or greases in order that the emulsion film or coating used is well adhered to the surface of the wire mesh 1. Generally, the metal fabric 1 may be cleaned by washing the same in a 5% to 10% glacial acetic acid solution for approximately 5 to 10 minutes and then rinsed well with hot water. The formulas for sensitizers may vary considerably and the most desirable type of sensitizer must be selected. One of the most eificient sensitizers found for use in the present invention contains ammonium bichromate, potassium bichromate, ammonium hydroxide, glycerin solution, and distilled water.

The metal fabric is then exposed to a proper light source through a photographic negative of the required print or design, such as shown in FIGURE 1. It can thus be seen by reference to FIGURE 1 that the metal fabric 1 is exposed to a proper white-light source 2 through a negative 3 having the proper design or image pattern which is to be formed on the metal fabric 1 such as illustrated by reference numeral 4. Generally, the metal fabric 1 may be suitably mounted within a desired frame or chase for ease of handling. The sensitized metal fabric 1 may then be exposed in either a dry or damp state depending on the type of screen that is being prepared. The length of the exposure is determined by experimentation but may vary anywhere from about 5 to 25 minutes depending on different factors. The type of light employed for exposing is very important and must be uniform in actinic intensity. In other words, the light energy should produce uniform chemical change in all areas during all times that the light is on. Generally, conventional photoflood lamps and are lamps provide suitable results.

After the sensitized metal fabric 1 has been exposed to the light source 2, it is then developed in a hardening or so-called developing tank 5 substantially as shown in FIGURE 2. Development generally takes place in hot Water with a temperature within the range of 95 to 115 F. The water preferentially washes away or dissolves all of the unexposed portions of the photosensitive emulsions. The portion of the emulsion residing in the image 4, which has been exposed to light becomes hardened and is now impervious to the wash treatment and remains on the metal fabric 1. The tank 5 is preferably made of glass, Bakelite, stainless steel, or a tank which is porcelain finished on the interior surface. The tank may be rocked gently forcing the rocking of the water to flow over the metal fabric 1 and provide some slight agitation.

After the developing operation in the tank 5, it can be seen that the sensitized metal fabric 1 has a positive image 4 on the surface thereof. It is often desirable for electrostatic printing techniques to build up a sufiiciently thick coating of an emulsion on the image area so that a sufficiently large relief may be obtained between the non-printing and the printing areas on the final electrostatic printing screen. Accordingly, the above operation for producing the positive image 4 on the surface of the screen may be repeated. In this process, a photosensitive emulsion is again applied to the surface of the screen by any of the suitable techniques aforementioned. Thereafter, the sensitized fabric 1 is exposed to the same light source or image pattern. In this case, however, the fabric 1 will have to be registered in position so that the image projected through the negative on the repeat operations is marginally registered with the positive image or pattern 4 on the surface of the fabric 1. After the sensitized fabric 1 is again exposed, it is hardened in the developing tank 5 so that the emulsion within the positive image pattern 4 is hardened and the remaining emulsion is washed away.

It has been found in connection with the present invention that a non-metallic fabric may be employed if the fabric is later gmetallized in some fashion. In the event that a non-metallic fabric is employed, the fabric is preferably removably mounted in a frame until the process of forming the fabric into a sufficiently rigid metal structure has been completed. The fabrics which may be employed in the construction of the electrostatic printing screens are silk, organdy, cotton, nylon, Dacron, vinyl, Vinyon, linen, etc. The fiber of the fabric may be made of animal, plant, mineral synthetic material or combinations of the above materials. However, of all of the fabrics attempted for use, a synthetic cloth containing monofilament strands such as cloth sold under the trademark nylon has been found to be most suitable. The cloth fabrics employed may be gauze weave, a leno weave, or a.plain weave. The gauze weave and leno weave are preferred since both produce relatively strong cloths. This type of fabric is then preferably metalized in a suitable metalizing chamber such as by vacuum metal vapor deposition process techniques. Thereafter, the process of producing electrostatic screens of the present invention is the same as one originally starting with a metal fabric.

The fabric 1 is next formed into a desired shape. The metal fabric 1, which is fairly thin still has sufiicient rigidity to be formed by hand, and to maintain its shape. For purposes of illustrating the present invention, the metal screen has been shown to be fonmed into a truncated conically shaped element substantially as shown in FIGURE 3. As the ends of the fabric 1 are turned, they may be tack welded or soldered in the manner as shown 6 in FIGURE 3. In order to obtain the desired shape, the fabric 1 may be formed about a mandrel or container having a size or shape similar to the desired size and shape of an article which is ultimately to be printed. For example, if it was desired to electrostatically print on the side walls of disposable thin-walled containers, such a container or the mandrel therefor could be used as the die upon which the fabric 1 is formed into the desired shape.

The \metal fabric 1 can then be made into a rigid structure by suitable powder metallurgy techniques. The fabric 1 in the form of the truncated conically shaped element is then suitably mounted on a plug or inner mold 6 to be hereinafter described in more detail.

The metal fabric 1 which is mounted on the plug or soealled mandrel type mold 6, is then suitably introduced into a powder metal solution in the form of a slurry 7 contained within a suitable tank 8 in the manner as shown in FIGURE 4. The powder metal is preferably suspended in a desirable carrier, such as water or any similar inert material which will carry the metal powder and is capable of being removed therefrom by drying. The powders which are capable of use in the present invention are powders of iron, nickel, copper, etc. and almost any metal powder which is capable of being hardened by powder metallurgical techniques. It is also possible to use composite materials, such as metal-metal combinations as the metal powder. Such materials which may be used are tungsten-silver, tungsten-copper, molybdenum-silver, and molybdenumcopper, etc, The compositions may vary within wide limits according to the properties required in the finally finished product. For example, commercial tungstate-silver compositions may contain from 10 to 70% silver. Also, compositions containing tungsten and 10% silver are also commercially available and have a density of about 17.5 grams per cubic centimeter. It is also possible to employ metal compositions consisting of three metals such as tungsten-nickel-copper. For example, a commercially available tungsten-nickel-copper composition existing in 90% tungsten, 7.5% nickel and 2.5% copper is available. By varying the compositions, it is possible to produce a material having a desired density. It is also possible to employ metal-non-metal combinations, such as coppergraphite and bronze-graphite material. These composites are produced by mixing fine copper powders or bronze powders with 5 to 70% graphite powders. Also, copperzinc and copper-tin brasses are very suitable. In some cases, a suitable binder such as tar may be added to secure satisfactory coherence of the product. Thus, it can be seen that a wide range of metals and non-metals combinations are available for use in the powder metallurgy techniques.

It should be understood that the metal powder may be applied to the surface of the fabric 1 by any suitable technique, such as dipping or coating. Moreover, the metal may be applied by wet brushing or by use of a spatula, if desired. Moreover, the metal may be in the form of a thick slurry, which may be painted on. However, in any event, care should be taken to remove any excess material from the image area 4.

As thefabric 1 is introduced into the metal slurry 7, the metal particles will fill in the interstices of the nonprinting areas, but will not adhere to the area in the printing image 4 inasmuch as the latter has the photosensitive emulsion thereon. A momentary introduction into the metal suspension 7 is all that is necessary in order to completely fill the interstices of the non-printing areas. After the die 6 has been removed from the metal suspension 7, the fabric 1 is permitted to dry so that the Water carrier may be completely removed from the metal. It has been found that the screen may be dried as it stands right on the mandrel-mold 6 or may be removed therefrom as desired. Drying will take place at a faster rate if the screen is removed from the mold 6. It has been found that air drying for approximately 30 minutes at room temperature conditions is sufficient. However, the fabric 1 may be dried in an oven at approximately F. for approximately 10 to 15 minutes for complete drying.

The dried metal fabric 1 is then suitably mounted on a metal powder compacting apparatus 9 substantially as shown in FIGURE 5, the mold 6 forming a part thereof. The apparatus 9 is hereinafter described in more detail. However, it can be seen that the compacting apparatus 9 is suitably introduced into a high pressure fluid tank 10 and carries the metal fabric 1. The high pressure tank 10 may be of any conventional construction and is, therefore, not described in detail herein. The purpose of the compaction process is for the shaping of the powder in the metal fabric 1 to coherent bodies which have sufficient strength to permit handling and safe transfer to a sintering furnace. Moreover, the compaction process permits the metal powders impacted in the interstices of the fabric 1 to have a uniform density and other characteristics which are essential for satisfactory sintering. Furthermore, the high pressure techniques permit the metal powders to become permanently embedded in the interstices of the metal fabric 1. It has been found in connection with the present invention that desirable results are obtained if the metal fabric 1 is subjected to pressures within the range of to 100 tons per square inch until the metal powder is well compacted. The pressure required, however, is somewhat variable depending upon the particular metal powder employed and the metal fabric 1 which is employed.

After the metal powder has been suitably compacted in the interstices of the metal fabric 1, the fabric 1 in the form of its truncated conical shape is suitably placed in a sintering furnace 11, substantially as shown in FIGURE 7. The furnace 11 is conventional in its construction and is, therefore, not described in detail herein. However, it is pointed out that the furnace is constructed of a desirable ceramic material which is capable of withstanding the temperatures necessary for sintering of the metal powder contained in the metal fabric 1.

The theory of sintering is well known and is, therefore, not set forth in detail herein. However, it is pointed out that by sintering, the metal powders can be transformed without fusion to a compact material that exhibits a strength of the same order of magnitude as cast and wrought materials of corresponding composition. The powder compacts are sintered by heating to temperatures of about two-thirds of the absolute melting temperature and at least to the annealing temperature of the metal powder employed. With some metals, considerably higher temperatures are employed. For example, the refractory metals such as tantalum, molybdenum, and tungsten are sintered at a temperature very close to their melting points. In homogeneous systems, as well as in the mixtures of metals with non-metals, sintering is preferably formed in the absence of a liquid phase. In mixtures of different metals, sintering may be performed either in the absence or in the presence of a liquid phase.

It is well understood that satisfactory sintering depends upon close control of the sintering atmosphere as well as of the heating cycle, that is the rate of heating, maximum temperature, time of maximum temperature, and rate of cooling. However, there are a number of commercially available apparatus necessary for performing this function and they are neither illustrated nor described in detail herein. In the case of the present invention, it is preferred that sintering is performed either in a neutral atmosphere or a reducing atmosphere or in a vacuum. Such suitable neutral atmospheres may be in the presence of nitrogen, argon or helium. Suitable reducing atmospheres are carbon monoxide, hydrogen, disassociated ammonia, natural gas, coke-oven gas and partially 'burnt hydrocarbons. The metal fabric 1 remains in the sintering furnace 11 for a time which is determined as previously mentioned by the type of metal powder employed and the type of metal fabric 1 employed. After the sintering process has taken place, the furnace 11 is allowed to cool for ultimate removal of the metal fabric 1. The fabric 1, which has been sintered, is shown in cross section in FIGURE 10. -It can be seen that the powdered metal is completely dispersed throughout the interstices of the screen and forms a substantially rigid structure. The fabric 1 is thus formed into a suitable electrostatic printing screen 12. The screen 12 may be preferably provided with metal end rings 13 as shown in FIGURE 8 to produce the final screen 12 as shown in FIGURE 9. The metal end rings may be attached by soldering or tack Welding to the upper and lower margins of the conically-shaped fabric in the manner as shown in FIGURES 8 and 9.

It should be recognized that the excess photo-sensitive emulsion which originally resides in the image pattern 4 is removed during the sintering operation. Because of the high temperatures, the photosensitive emulsion is disintegrated, thereby leaving an open area in the image pattern 4. Furthermore, it should be understood that it is also possible to remove the photosensitive emulsion in the image pattern 4 by a differential etching solution. Any of a number of commercially available emulsion removers may be employed. It should also be recognized that the metal fabric 1 may be formed into double curvature and more complex shapes as desired by such operations as drawing, punching and molding at various stages of the preparation.

It is also possible to slightly modify the process of the present invention by eliminating the powder metallurgical technique and substituting therefor, the use of polymerizable plastics or various polymers, such as polycaprolactam. Other materials which may be employed are epoxy resins, plaster of Paris, porcelain cement, etc. In fact, any of a number of materials which are capable of polymerizing and hardening in the interstices of the metal fabric 1 can be suitably employed.

Some of the polymers which may be useful in the present invention are the polyalkenes formed of such monomers as ethylene, propylene, isobutylene, polydialkenes formed from monomers such as butadiene and isoprene, the halogenated polyalkenes formed from monomers such as tetrafluoroethylene, vinyl resins such as polyvinylacetal, polyvinylacetate, polyvinylchloride, polystyrenes formed from such monomers as styrene, etc. Another class of synthetic polymers capable of being used in the present invention are those of the cellulose derivatives, e.g. cellulose esters such as cellulose acetate, cellulose triacetate, cellulose propionate and acetal resins such as those formed by addition polymerization of formaldehyde or higher aldehydes. Also useful are the phenolic resins formed by the condensation of phenol, cresol, xylenol, and other substituted phenols with formaldehyde or higher aldehydes. Other polymer systems which can be used are those obtained by the polymerization of furfural or furfuryl alcohol and by copolymerization of furfural and a ketone. Various other polymer groups are also useful in the present invention such as the polyurethane resins prepared by the reaction of polyols, polyesters, etc. and polymers of the polyamide type produced by condensation of diamines with dicarboxylic acids. The polymers may be linear or crosslinked.

It has been found that screens of this type find particular utility for electrostatic printing in the type of apparatus described in my copending application Ser. No. 472,982, filed July 19, 1965. In the apparatus and method described in said copending application, an electrostatic printing screen is disposed circumferentially around the article to be printed, which is generally a disposable plastic thin-walled container. Generally, the substrate has a curvilinear shape, such as a comically-shaped container and the counter-electrode in the electrostatic printing apparatus is generally in the form of a mandrel for sup porting the container and which has the same size and shape as the container. Thus, the container is mounted on the mandrel in spaced relation to the screen. In the case of electrostatic printing with the screen of the present invention, the screen is circumferentially disposed about the container and in close proximate relationship thereto,

where ink can be applied to the open mesh portions of the screen 12 and deposited on the surface of the container. It has also been found that electrostatic printing can be performed on curvilinear surfaces where the screen has a surface contour in at least one plane which is substantially identical to at least one portion of the surface contour of the element being printed, such as in the method described in my copending application Ser. No. 472,982, filed July 19, 1965. In this latter process, the screen is rotated or oscillated in timed relation to the rotation or oscillation of the element being printed.

In this manner, the container surface tangentially approaches and tangentially departs from the screen so that printing will occur through the screen along an elemental line of closest approach. The container is generally rotated at approximately the same rate of speed as the rotation or oscillation of the screen through this line of closest approach so that a continuing line of tangency occurs between the surface of the container and the surface of the screen. A desired electroscopic ink is fed through the screen along this line of closest approach by any suitable ink feeding mechanism. Furthermore, by use of screens produced by the method of the present invention, it is possible to employ contact electrostatic printing on curvilinear surfaces. This method is again described in the aforementioned copending application.

Any of a variety of electroscopic inks can be used Where the screen produced by the present invention is used in the electrostatic printing operations. Generally, the electroscopic inks comprise a finely dispersed powder which is capable of being triboelectrically charged. The powder generally carries a desired pigment. A number of satisfactory powders can generally be employed in the electrostatic printing operations and each must be in a finely divided state. Suitable powders are dyed thermoadhesive resins such as rosin, gum copal, gum sandarac, ethyl cellulose, Egyptian asphalt and the like. A very satisfactory thermoadhesive powder can be produced by dissolving equal parts of ethyl cellulose and vinsol resin in acetone together with a small amount of spirit soluble aniline dye such as'Nigrosine or aniline blue and spray drying the solution to produce an extremely fine powder having substantially spherical particles. Dyed Lycopodium powder is suitable where thermoadhesive properties are not required of the powder, as is also starch, cellulose flour, powdered metal and copper powder.

Whether fusible, thermoadhesive or non-fusible powders or others are used, the particle size is preferably near the limit of definition of the eye under ordinary reading conditions. Excessive powder size contributes to graininess in appearance of the image. n the other hand, extremely fine powder may be undesirable in many instances due to its tendency to ball up or cling together in clusters. It is, therefore, desirable to use a powder in which substantially all the particles are within the size range of 2 to microns. If spherical powders are used, this refers to their diameters, otherwise to the largest dimension. For most purposes, it is preferred to use an equidimensional powder particle, the sphere being the preferred form.

As previously indicated, when the screen produced by the present invention is employed in an electrostatic printing operation, voltage is applied to the counterelectrode which holds the substrate, the screen and to a feeding electrode. The charge in the counter-electrode is opposite to the charge on the screen so the particles are propelled through the screen openings or interstices of the screen toward the counter-electrode. The image is then generally subsequently fixed by heat, a solvent or a vapor, or by any other suitable means depending upon the type of pigment powder which has been employed and the nature of the material being printed.

The current source employed in electrostatic printing operations is adapted to develop a relatively high direct current potential. While the current requirements for electroprinting of the type here employed are not heavy in the ordinary sense, a very definite electron current or space current flows across the printing space during the printing operation. It is desirable to have a space current of at least 1 to 2 milliamperes per square inch of printing area. Moreover, the high potential source should be capable of maintaining a desired voltage under current range in the range of approximately milliamperes or slightly more.

It should be recognized that the electrostatic screens produced by the present invention have the capabilities of withstanding this type of high voltage potential applied thereto. Moreover, they are capable of withstanding abrasion of the rough handling by virtue of their construction.

The compaction apparatus 9 illustrated in FIGURES 5 and 6 may be considered as a molding press and generally comprises the mandrel type die or plug 6 which generally has the surface contour similar to the surface contour of the screen being produced. In the case of the present invention, for example, the plug 6 has a surface contour, shape and size which is substantially similar to the mandrel upon which the container to be printed would be mounted and substantially similar to the screen '12. Suitably disposed around the outer surface of the plug 6 is a somewhat flexible but sufficiently rigid pressure forming die 14. The die 14 is secured at its upper and lower ends to the exterior surface of the plug 6 by means of any suitable adhesive, such as an epoxy resin 15, thereby forming a suitable air space 16 between the pressure forming die 14 and the plug 6. The plug 6 is provided with a hollow fluid chamber 17, which has a series of fluid apertures 18 in its side wall providing communication between the air space 16 and the fluid chamber 17. Morever, the plug 6 is provided on its upper surface, reference being made to FIGURE 5 with a series of fluid ports 19 providing communication between the fluid chamber 17 and an exterior atmosphere such as the interior of the pressure tank 10. The plug 6 is also provided with a stem or handle 20 in the form of a tube which extends through the bottom wall of the plug 6 and communicates with the space between the exterior surface of the plug 6 and the pressure forming die 14. The stem 20 is also provided with a conventional type of valve 21 which may be opened and closed as desired for removing air from the space 16.

The plug 6 is preferably constructed on any rigid metal such as stainless steel, aluminum and should be capable of withstanding the high pressures normally encountered in compaction operations in powder metallurgy techniques. The pressure forming die 14 is preferably formed of any rigid rubber material which is capable of providing sufiicient flexibility when subjected to pressures within the range of 5 to 100 tons per square inch. It has been found that suitable rubber materials which may be employed are neoprene rubber, polyvinyl butyral, Buna-N rubber, sodium polybutadiene, and many naturally occuring rubbers. Many synthetic rubber polymer combinations such as butadiene and styrene have also proved suitable for forming the pressure forming die 14. Butyral rubber, produced by the polymerization of isobutylene and a small amount or isoprene at approximately F. in the presence of a Friedel-Crafts catalyst has also been found to be useful. Similarly, special rubbers consisting of copolymers of butadiene and acrylonitrile have proved to be very useful. Additionally, some polysulfide and silicon rubbers which present sufficient rigidity and yet sufiicient flexibility for positive pressure forming have been found to be useful for the purposes of the present invention.

Disposed around the positive pressure forming die 14 is the metal fabric 1 which is in the form of the screen 12 and disposed about the fabric 1 is a female positive pressure forming die 22 which has the same surface contour and shape as the fabric 1 and the die 14. Thus, it can be seen that the fabric 1 is held between the two pressure forming dies 14 and 22. Moreover, the pressure forming die 22 is formed with the same flexibility as the die 14 and of the same material and same thickness. It has been found in connection with the present invention that when each of the dies 14, 22 is formed with an overall thickness of approximately 0050-0125", very suitable results have been obtained. Each of the elements, namely the plug 6 with the positive pressure forming die 14 secured thereto, the fabric 1 and.the die 22 are all secured by means of a releasable clamp 23. The band 23 is preferably provided with a releasable tightening mechanism such as a screw type clamp, where each of the aforementioned elements can be assembled in the manner as shown in FIGURE 5. In this manner, a pressuretight air seal is maintained between the pressure forming dies 14 and 22.

.In' use, the fabric 1 is disposed about the pressure forming die 14 and the outer pressure forming die 22 is disposed around the fabric 1 and each of the elements is secured by means of the removable clamp 23. Thereafter, the area between the two dies 14, 22 is evacuated by opening the valve 21 and connecting the stem 20 to a suitable vacuum. Evacuation of this area occurs through the stem 20. After a sufiicient quantity of air has been removed from this chamber, the valve 21 is closed. Thereafter, the entire assembly is inserted into a pressure vessel, such as the tank 10 which is closed and subjected to high hydrostatic pressure. It can be seen that the fabric 1 is maintained in a fluid-tight chamber between the two pressure forming dies 14, 22. The fluid under pressure will flow through the fluid ports 19 into the fluid chamber 17 and through the apertures 18 into the air space 16. As this occurs, the fluid under pressure will force the male pressure forming die 14 into contact with the interior surface of the conically shaped fabric 1. Pressure against the outer surface of the female positive pressure forming die 22 will force the same into contact with the exterior surface of the fabric 1, thereby compacting the metal powders into a solid homogeneous metal fabric. After the compaction operation has been completed, the entire assembly may be removed and the valve 21 opened, permitting disassembly and removal of the metal fabric 1.

It should be understood that changes and modifications in the form, construction, arrangement and combination of parts presently described and pointed out may be made 12 and substituted for those herein shown Without departing from the nature and principle of my invention.

Having thus described my invention, what I desire t claim and secure by Letters Patent is: Y

1. The method of making electrostatic printing screens having opened and closed areas for use in electrostatic printing processes, said method comprising metallizing a non-metallic fabric, applying an image pattern to said fabric creating image and non-image areas, bending said fabric to produce a desired cross-sectional shape, filling the interstices of the non-image areas of the fabric with a metal containing powder, pressurizing the powder in the fabric to cause the powder to become impacted in the fabric interstices, and heating the fabric thereby causing the metal to become sintered, thereby creating a substantially rigid electrostatic printing screen.

2. The method of making electrostatic printing screens having opened and closed areas for use in electrostatic printing processes, said method comprising metallizing a non-metallic fabric, applying a photosensitive emulsion to said fabric, exposing the emulsion with an image pattern interposed between the fabric and the means of exposing, removing the non-exposed portions of the photosensitive emulsion thereby creating image and non-image areas, bending said fabric to produce a desired cross-sectional shape, filling the interstices of the non-image areas of the fabric with a hardenable material, removing the remaining photosensitive emulsion, and causing the material to harden thereby creating a substantially rigid electrostatic printing screen.

References Cited UNITED STATES PATENTS 2,213,237 9/1940 Brennan et al. l0l128.2 X 2,288,020 6/1942 Noland et al 101-1282 2,573,951 11/1951 Brennan 101128.2 2,860,576 11/1958 Short 9636.4 X

FOREIGN PATENTS 756,315 9/1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

R. E. MARTIN, Assistant Examiner.

U.S. Cl. X.R.