US2860576A - Method of producing stencil screens - Google Patents

Description

United States Patent METHOD OF PRODUCING STENCIL SCREENS Oliver A. Short, Metuchen, N. J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 24, 1956 Serial No. 586,923

3 Claims. (Cl. 101-1283) This invention relates to the production of stencil screens, and more particularly it relates to a method of producing such screens that will resist elevated temperatures for extended periods of time.

Stencil screens for the application of inks, colors, ceramic pigments, and the like have been used for many years. Stencil screens generally used in the ceramic industry are woven with fine metal, preferably stainless steel, wire to a mesh size of between about 150 to 175 meshes per linear inch. Such screens are prepared for printing by closing all of the interstices between wires except those which define the design which is intended to be printed. The design is printed on a surface by forcing semi-fluid or pasty printing material through the remaining open meshes by means of an instrument commonly referred to as a squeegee.

These metal screens may be prepared for printing of simple designs by applying cellulosic or similar films, from which the designs have been cut, onto the bottom surface of the screens. Where the screens are to be pre pared for printing more complicated designs or indicia, the screens are commonly coated with a uniform film of a photo-sensitive material which is dried in complete darkness and then exposed to strong light through a positive photographic film of the design or indicia. After exposure, the film protected by the design on the photograph remains soluble and may be washed from the screen with a liquid, usually water, thus forming the open meshes of the screen. The remaining portions of the film are rendered insoluble by the actinic rays, thus forming the closed screen meshes.

More recently, in the ceramic industry, ceramic colors are applied in a heated, molten state through stencil screens. In this process the ceramic colors are dispersed in a wax or wax-resin vehicle which is solid at room temperature but semi-liquid or pasty at a temperature of between about 120 to 122 C. The ceramic colors are molten and maintained in the molten state on the stencil screen by application of heat. The heat may be transmitted to the screen either by radiation, such as by infra-red rays, by conduction or induction, such as by conducting or inducing an electric current through the stencil screen wires.

Such applications of heat to the stencil screens very materially shortens the screen life due to the poor heat resistance of the gelatin or polyvinyl alco-rol films, heretofore used for this purpose, on the screens.

It is, therefore, an object of this invention to provide a new and improved method of closing, or filling, the desired meshes of a stencil screen with a heat-resistant organic film.

It is another object of this invention to close or fill the desired meshes of a stencil screen with a heat-resistant organic film by a simple method which will permit the formation of complicated design on the screen.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by first forming on a stencil screen a photographic negative of the desired design with filled meshes 2. of a soluble, light-hardened material, filling the open screen meshes of such design with a heat-resistant organic film taken from the group consisting of polytetrafluoroethylene and polysilicone resins, drying said heat-resistant organic film to insolubilize the same, and dissolving from the screen said soluble, light-hardened material, thus leaving on the screen a photographic positive of the desired design with filled meshes of a heat-resistant, insoluble polytetrafluoroethylene or polysilicone.

In forming on the stencil screen a photographic negative of the desired design with filled meshes of soluble light-hardened material, any material may be used that in its light-hardened state after exposure to light in the presence of a light-sensitizing agent will be soluble in a liquid which will not dissolve the subseqeuntly applied film of polytetrafluoroethylene or polysilicone. Substantially all commonly used light-sensitized, film-forming materials may be used for this purpose. Such material may be composed of polyvinyl alcohol, water-soluble gelatin, agar-agar, polyglycuronic acid, albumin, watersoluble ethyl cellulose, water-soluble polyvinyl acetals, and the like, sensitized to light with conventional lightsensitizing materials such as light-sensitive silver halides, light-sensitive bichromates, diazonium compounds, and the like.

The light-sensitized, film-forming material in aqueous solution is applied as a uniform film on the surface of the stencil screen and dried in the darkroom. The dried film is then exposed to a strong light through a negative photographic film or plate until the light passing through the negative has water-insolubilized the portions of the film so exposed. The remaining soluble portions of the film are then washed from the screen by spraying with cold or warm water as required. The screen is now.

provided with the desired design in photographic negative with open and filled meshes.

The open meshes are then filled with an aqueous dispersion of polytetrafluoroethylene or with an ether solution of a heat-resistant polysilicone which is then dried to set the dispersion or solution in the form of a highly heat-resistant solid that is insoluble in most common solvents.

The light-hardened, water-insoluble portions of the light-sensitized film are then removed by treatment with a solvent or solution that will dissolve the same without dissolving the applied polytetrafluoroethylene or polysilicone. In marginal portions of the screen design where the polytetrafiuoroethylene or polysilicone overlies portions of the light-sensitized film the former will easily be removed by a strong water spray after the light-sensitized film is dissolved from the screen.

The heat-resistant polysilicones useful in carrying out the process of this invention are the polysilicones free from carbon-to-carbon bonds as disclosed in Rochow Patents Nos. 2,258,218, 2,258,219, 2,258,220, and 2,352,974. Polytetratluoroethylene is a resin commonly known to be highly heat resistant up to 350 C. and is available in 40% to 60% solids aqueous dispersion under the trade-name Teflon from I. do Font de Nemours & Company, Wilmington, Delaware.

The heat-resistant polytetrafluoroethylenes useful in carrying out the processes of this invention are disclosed in the following patents: Berry 2,478,229, Sanders 2,520,173, and ()sdal 2,562,117 and 2,562,118.

The following examples are given to illustrate in detail specific methods for the production of stencil screens of this invention. The details set forth in these examples are not to be taken as limitative of the scope of this invention.

Example I A screen stencil of 10 inch by 20 inch dimension comprising woven, stainless-steel wire with 165 meshes per linear inch stretched on an aluminum frame is dipped into a solution of a light-sensitive coating composition consisting of the following:

2002. polyvinyl alcohol, a 4% solution in water having a viscosity of 20-25 centipoises; lgal. water; and 8 /2 oz. potassium bichromate solution (saturated).

This dipping is carried out so that the coating composition lightly contacts the screen surface and does not seep through the mesh. The coated screen is then dried, at a temperature of 8590 F, in complete darkness.

'Nextplace a photographic negative film of the desired design over the applied film and direct a 35 ampere,

1500 candle power light toward said negative from a dis tance of 36 inches for a period of about five minutes. After such exposure, the polyvinyl alcohol film protected by the design of the photographic negative will remain soluble while the remainder of the film has been rendered water-insoluble by the actinic rays of the light. The screen is then immersed in lukewarm water and then sprayed with water at room temperature, the spray being directed toward the film side of the screen. The soluble polyvinyl alcohol is thus removed from the screen.

The screen is then dried and a 60% aqueous dispersion of polytetrafluoroethylene is applied by brushing onto the open mesh portion of the screen. The screen is then dried at a temperature of 100 C. and heated for ten minutes to a temperature of 200 C. to sinter the particles of polytetrafluoroethylene and form a continuous film over the covered meshes.

The light-insolubilized polyvinyl alcohol is then removed from the screen by first soaking and then spraying the film side of the screen with an aqueous hydrogen peroxide solution containing 35% H 0 In this form the screen now has the open mesh in the desired positive pattern. The screen is then heated for ten minutes at a temperature of 375 C. to harden the polytetrafiuoroethylene. The resulting screen may be used at elevated temperatures for printing molten thermofluid ceramic colors over long periods of time.

Example II 4% Patent No. 2,258,218. The screen is then heated and dried at a temperature of 200 C., after which the insolubilized gelatin is removed therefrom by washing and spraying with a gelatin solvent.

The resulting screen may be used in a squeegee printing process at a temperature up to 250 C. for long periods of time without damage to the silicone masking material.

Throughout the specification and claims, any reference to parts, proportions, and percentages refers to parts, proportions, and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. The process of incorporating a design on a stencil screen which comprises forming thereon a photographic negative design with a soluble light-hardened material, filling the open screen meshes of such design with a liquid organic film taken from the group consisting of polyvtetrafluoroethylene and polysilicone resins, drying said film to form a heat-resistant insoluble solid, and dissolving said soluble light-hardened material from said screen, thus leaving on the screen a photographic positive of the desired design with filled meshes of said heat-resistant material.

2. The process of incorporating a design on a stencil screen which comprises forming thereon a photographic negative design with a soluble light-hardened material, filling the open screen'meshes of such design with a liquid film of polytetrafluoroethylene, drying said film to form a heat-resistant insoluble solid, and dissolving said soluble light-hardened material from said screen, thus leaving on the screen a photographic positive of the desired design with filled meshes of said heat-resistant material.

3. The process of incorporating a design on a stencil screen which comprises forming thereon a photographic negative design with a soluble light-hardened material, filling the open screen meshes of such designwith a liquid film of a polysilicone resin, drying said film to form a heat-resistant insoluble solid, and dissolving said soluble light-hardened material from said screen, thus leaving on the screen a photographic positive of the desired design with filled meshes of said heat-resistant material.

References Cited in the file of this patent UNITED STATES PATENTS 1,064,166 Pindikowsky June 10, 1913 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent'No, 2,860,576 November 18, 1958 Oliver A., Short It is'hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below,

Column 1, line 50, for ''122 C0" read 212 C0 line 57, for "applications" read application line 59, for "alcorol" read alcohol Signed and sealed this 10th day of March 1959,

(SEAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Officer Comr'xissioner of Patents