Electrostatographic process for preparing screen printing member
United States Patent 3909256
A novel method for preparing a stencil master is provided. The method comprises coating the voids of a suitable screen with a liquid but hardenable material which is impervious to a printing fluid and not dissolvable by said printing fluid and which has a viscosity and conductivity to permit removal by electrostatic attraction, contacting said screen in the absence of light with a member having an electrostatographic latent image, separating the screen and imaged member whereby the liquid is removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration and hardening the liquid in the noncharged areas to form an imaged stencil master.
US Patent References:
Means for printing
Huebner - September 1946 - 2408144
Stencil making
Matt - August 1960 - 2949848
Stencil master making
Gundlach - August 1960 - 2949849
FROST GRAVURE PRINT MASTER
Gundlach et al. - March 1969 - 3445226
METHOD OF PREPARING AND USING A GRAVURE PRINTING PLATE
Martel et al. - February 1971 - 3559570
Means for printing
Huebner - September 1946 - 2408144
Stencil making
Matt - August 1960 - 2949848
Stencil master making
Gundlach - August 1960 - 2949849
FROST GRAVURE PRINT MASTER
Gundlach et al. - March 1969 - 3445226
METHOD OF PREPARING AND USING A GRAVURE PRINTING PLATE
Martel et al. - February 1971 - 3559570
Application Number:
05/427591
Publication Date:
09/30/1975
Filing Date:
12/26/1973
View Patent Images:
Export Citation:
Assignee:
Xerox Corporation (Stamford, CT)
Primary Class:
Other Classes:
427/483, 101/128.400, 427/468, 101/128.210
International Classes:
G03F5/20; G03F7/12; G03G13/32; G03F5/00; G03G13/26; G03G13/26
Field of Search:
96/1R,1LY 117/37LE,35.5 101/128.2,128.3,128.4
US Patent References:
| 3561358 | GRAVURE IMAGING SYSTEM | February 1971 | Weigl | |
| 3589289 | PRINTING MEMBERS AND METHODS FOR GRAPHIC COMPOSITION | June 1971 | Gosnell et al. | |
| 3676215 | June 1972 | Gundlach | ||
| 3687072 | ELECTROSTATIC COPYING | August 1972 | Pym | |
| 3795530 | March 1974 | Gundlach | ||
| 3801315 | GRAVURE IMAGING SYSTEM | April 1974 | Gundlach et al. | |
| 3806354 | April 1974 | Amidon et al. |
Primary Examiner:
Torchin, Norman G.
Assistant Examiner:
Miller, John R.
Attorney, Agent or Firm:
Ralabate, James O'sullivan James Mackay Donald J. P. M.
Claims:
What is claimed is
1. A method for preparing a stencil comprising providing a suitable screen, coating the voids of said screen with a liquid material hardenable at ambient or elevated temperature which is impervious to a printing fluid and not dissolvable by said printing fluid and which has a viscosity and conductivity sufficient to permit removal by electrostatic attraction, said conductivity being less than 1013 ohm/cm, contacting said coated screen in the absence of light with a member having an electrostatographic latent image separating the members whereby liquid is removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration and hardening the liquid in the noncharged areas to form an imaged stencil.
2. The process of claim 1 wherein the voids of the screen are coated with a silicone.
3. The process of claim 1 wherein the image on the image member is xerographically formed by charging a photoconductive layer and exposing said layer to imagewise light to form an electrostatographic latent image.
4. The process of claim 1 wherein the image member comprises zinc oxide photoreceptive paper.
5. The process of claim 1 wherein the screen is a metal screen.
6. The process of claim 1 wherein the screen is formed of stainless steel.
7. The process of claim 1 wherein the screen has a mesh size of from 60 to 325 U.S. mesh.
8. The process of claim 1 wherein the screen has a mesh size of from 100 to 250 U.S. mesh.
9. A method of printing comprising providing a suitable screen, coating the voids of said screen with a liquid material hardenable at ambient or elevated temperature which is impervious to a printing fluid and not dissolvable by said printing fluid and which has a viscosity and conductivity sufficient to permit removal by electrostatic attraction, said conductivity being less than 1013 ohm/cm, contacting said coated screen in the absence of light with a member having an electrostatographic latent image, separating the members whereby the liquid is removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration and hardening the liquid in the nonimaged areas to form an imaged stencil, contacting the resultant stencil with an image receiving member and passing ink through said voids to thereby form an inked image on said receiving member.
10. The method of claim 1 wherein the screen is coated with the liquid material, and doctored with a flexible blade to remove the liquid from the contact plane so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen only by electrostatic attraction.
11. The method of claim 1 wherein the electrostatographic latent image member contains an image which is negative in sense and the screen is biased to a potential which is approximately the same as those areas of the image member which are of the higher potential so that when the members are contacted and separated, an image is provided which is positive in sense.
12. The method of claim 9 wherein the screen is coated with the liquid material, and doctored with a flexible blade to remove the liquid from the contact plane so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen only by electrostatic attraction.
1. A method for preparing a stencil comprising providing a suitable screen, coating the voids of said screen with a liquid material hardenable at ambient or elevated temperature which is impervious to a printing fluid and not dissolvable by said printing fluid and which has a viscosity and conductivity sufficient to permit removal by electrostatic attraction, said conductivity being less than 1013 ohm/cm, contacting said coated screen in the absence of light with a member having an electrostatographic latent image separating the members whereby liquid is removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration and hardening the liquid in the noncharged areas to form an imaged stencil.
2. The process of claim 1 wherein the voids of the screen are coated with a silicone.
3. The process of claim 1 wherein the image on the image member is xerographically formed by charging a photoconductive layer and exposing said layer to imagewise light to form an electrostatographic latent image.
4. The process of claim 1 wherein the image member comprises zinc oxide photoreceptive paper.
5. The process of claim 1 wherein the screen is a metal screen.
6. The process of claim 1 wherein the screen is formed of stainless steel.
7. The process of claim 1 wherein the screen has a mesh size of from 60 to 325 U.S. mesh.
8. The process of claim 1 wherein the screen has a mesh size of from 100 to 250 U.S. mesh.
9. A method of printing comprising providing a suitable screen, coating the voids of said screen with a liquid material hardenable at ambient or elevated temperature which is impervious to a printing fluid and not dissolvable by said printing fluid and which has a viscosity and conductivity sufficient to permit removal by electrostatic attraction, said conductivity being less than 1013 ohm/cm, contacting said coated screen in the absence of light with a member having an electrostatographic latent image, separating the members whereby the liquid is removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration and hardening the liquid in the nonimaged areas to form an imaged stencil, contacting the resultant stencil with an image receiving member and passing ink through said voids to thereby form an inked image on said receiving member.
10. The method of claim 1 wherein the screen is coated with the liquid material, and doctored with a flexible blade to remove the liquid from the contact plane so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen only by electrostatic attraction.
11. The method of claim 1 wherein the electrostatographic latent image member contains an image which is negative in sense and the screen is biased to a potential which is approximately the same as those areas of the image member which are of the higher potential so that when the members are contacted and separated, an image is provided which is positive in sense.
12. The method of claim 9 wherein the screen is coated with the liquid material, and doctored with a flexible blade to remove the liquid from the contact plane so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen only by electrostatic attraction.
Description:
BACKGROUND OF THE INVENTION
In stencil printing a porous-filled sheet is opened up where it is desired to make an ink flow for imaging; no ink can pass through unopened, blocked areas. Stencil printing includes both screen process printing and mimeograph duplicating. Stencil screens can be made by photoresists. For example, gelatin can be coated on a screen, sensitized by dichromate solution, dried, exposed and washed out to give an imaged screen. Ink can pass through where light has not struck. In mimeograph duplicating, imaged stencils consist essentially of ink-impervious coatings cut to expose a permeable sheet that permits ink to pass through. The stencils are wrapped around a rotating drum of a duplicator. This drum is perforated to permit ink brushed through it to pass to the stencil to image paper fed against the stencil on the drum.
In general, the processes for producing stencils are costly and time consuming. It is to this problem that this invention is directed.
DESCRIPTION OF THE INVENTION
It has now been discovered that stencils can be prepared rapidly by an inexpensive simple camera speed imaging process. More particularly, it has been found that when the voids of a screen are filled by coating with a liquid but hardenable material which is impervious to a printing fluid, not dissolvable by said printing fluid and which has a viscosity and conductivity to permit removal by electrostatic attraction, the screen can be contacted in the absence of light with a member having an electrostatographic latent image and the members separated to cause the liquid to be removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration, and the liquid can be hardened in the remaining areas to form an imaged stencil master. Thus because the stencil is formed from a screen, it is not necessary to use etch solutions in order to form the image but rather the liquid can be removed from the screen to provide voids by simple electrostatic attraction. In addition, it is not necessary to form light sensitive coatings on the screen as the image can be easily and rapidly formed on a photoconductive paper such as a conventional zinc oxide master and the screen imaged by contacting the charged master with the screen and separating the members to remove the liquid from the screen in image configuration. Further, the stencils can be prepared from conventional materials and employed for both screen process printing and mimeograph duplicating. Other benefits will be apparent from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
Screens which can be employed for the printing stencil include mesh, fabric or cloth formed of any weavable material such as silk, nylon, Dacron and the like fibers as well as metal wires such, for example, as stainless steel. The screen will preferably have from 100 × 100 to 250 × 250 U.S. mesh, although larger and smaller screens can be employed.
Liquid materials suitable for filling the screen member include waxes, and a variety of natural polymerizable monomers and synthetic polymers which are of a low viscosity or can be made of a low viscosity. It is only necessary that the material be sufficiently liquid when applied to the screen such that it fills the voids, that the material can be hardened such that it is impervious to the printing fluid employed and not dissolvable by the printing fluid, and that the fluid can be removed from the screen by electrostatic attraction. Thus it can be appreciated that a large number of materials can be employed.
The liquid but hardenable material can be removed from the screen member by capillary action even if the viscosity is high when the members are contacted with the imaging member for a relatively long time. It is preferred, however, that the liquid material have a sufficiently low viscosity and resistivity that they can be quickly removed by use of an electrostatic field. Nearly all of the materials are sufficiently conductive as it is only necessary to have a resistivity of less than 10 13 ohm/cm and for the few materials which are below this conductivity, a common electrolyte such as salt can be added. Likewise, the viscosity is not a critical parameter, it only being necessary for optimum results that the liquid material and ink can be selectively removed by an electrostatic field. In order to obtain the desired viscosity, the monomers or polymers can be dissolved in a suitable solvent in which they are soluble, as is well known to those skilled in the art.
The liquid materials which can be employed to fill the screen member include materials which are hardenable at ambient temperature, or elevated temperature and which can be hardened or cured by cooling, air, or some form of radiant energy such as heat and ultraviolet light. Photo-hardenable polymers which can be cured by light are particularly preferred as it is not necessary to adjust the temperature of the screen member. The particular liquid employed will depend on the desired life of the screen member, the substrate to which it is to be adhered and other variables such as cost and availability.
Waxes can be employed which are solid at room temperature and can be made liquid at elevated temperature. Representative examples include: beeswax, carnauba wax, parafin waxes and the like. Preferred waxes are those melting below 100°C and most preferably below 60°C.
Liquid polymers which can be employed to fill the screen member include low viscosity thermoplastics, thermosetting plastics thermosetting resins and photopolymers. Exemplary of suitable thermoplastics are polyethylene, polypropylene, ethylene-vinyl acetate copolymers, propylene-modified polyethylene, acetals, acrylics, acrylonitrile-butadiene-styrene (ABS), polystyrene, cellulosics, shellac, chlorinated polyether, fluorochemicals, polyamides (nylons), polyimides, phenoxies, and vinyls. examplary of suitable thermosetting plastics are aminoplasts (urea-formaldehyde, melamine-formaldehyde), phenolics, epoxies, diphenyl oxide, polyurethanes, polyesters, diallyl phthalates, and silicones. Exemplary of thermosetting resins are vinyl, acrylic, alkyd, polyurethane, silicone, phenolic, and epoxy resins.
Polymerizable monomers can also be employed to fill the screen member. For example, monomers such as methyl, ethyl and propyl methacrylate, styrene and pivalolactone can be employed with an initiator. Initiators which can be employed to polymerize the aforesaid monomers (other than pivalolactone) include conventional materials such as for example, benzoyl peroxide, and azo-bis(isobutyronitrile). An initiator for pivalolactone is triphenyl phosphine. A number of materials can be employed and it is only necessary that the initiator or catalyst be soluble in the monomer or very finely dispersed.
Exemplary of suitable photopolymers are the cinnamic resins of polyvinyl alcohol, cellulose, starch and the epoxy resin of epichlorohydrin and 4,4'-isopropylidenediphenol. Polymethacrylate and polyamide coatings can also be employed when mixed with photosensitive polymerizable materials.
The screen member can be filled by conventional means. For example, it may be dipped in the liquid material or draw bar coated.
The thermoplastic materials and waxes which are solid at ambient temperature can be converted to a low viscosity by heat so as to render them electrostatically removable. The thermosetting polymers are liquid at ambient temperature or below their curing temperature and the photopolymers are liquid until cured such as by ultraviolet light.
If desired, a screen member can be prepared by filling the voids of the screen member at elevated temperature with a wax or thermoplastic material and then allowing the material to harden by cooling. When it is desired to image the screen member, the material can be softened by heat to fluidize the material so as to permit removal in the charged areas by electrostatic attraction.
In order to prevent the removal of the liquid material from the screen network (i.e. threads or lines of the screen) and from the interstices in the nonimaged areas because of physical contact when the screen and imaging member are contacted and separated, it may be necessary that the screen be doctored, after the voids are filled and before imaging, with a flexible doctor blade such as soft rubber, nylon or polyurethane. In this manner, the fluid is removed from the lands, and below the contact plane of the interstices so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen by electrostatic attraction in the charged areas of the image and not from the noncharged areas by physical contact. Whether or not it will be required to doctor the screen depends upon the method of application of the liquid material, its consistency and affinity for the screen network.
The electrostatographic image can be formed by conventional means such as xerography. By this method the image is formed on a photoreceptor imaging member such as a zinc oxide paper master or a selenium drum. Then the electrostatographic latent image on the imaged member is contacted to the coated screen in the absence of light and the two members quickly separated. The contact and separation of the two members causes the liquid to be removed from the screen in areas corresponding to the charged areas. The liquid remaining in the noncharged areas on the screen can then be hardened by heat, air, ultraviolet light or the like, the particular treatment depending on the material employed as the liquid. Images of either positive or negative sense can be formed.
In addition to the photoreceptors mentioned, other conventional photoreceptors can be employed. Typical photoreceptors include inorganic materials such as cadmium sulfide, cadmium sulfoselenide, mercuric sulfide, lead oxide, lead sulfide, cadmium selenide and mixtures thereof dispersed in binder or as homogeneous layers. Typical organic photoreceptors include pigments such as quinacridones, carboxanilides, triazines and the like. The electrostatographic latent image can be transferred directly from a photoreceptor to the screen member or it can be first transferred to a dielectric material such as Mylar polyester and the dielectric material contacted with the screen member. Further, the electrostatographic image can be formed without the use of a photoconductor by applying a metal stencil over a dielectric member such as a polyester film and the surface charged according to the pattern in the stencil. The charged film is contacted to the screen member, separated and the fluid hardened in the nonimage areas as described above.
When it is desired to make a positive image from an image which is negative in sense, one may charge and expose a photoconductor to activating electromagnetic radiation to provide a negative image and bias (charge) the screen to a potential which is approximately the same as those areas of the image which are of the higher potential so that when the two members are contacted and separated, the nonimage areas are developed to provide a stencil with an image which is positive in sense.
Typical inks and printing equipment can be employed with the stencil master of the invention. Typical inks include inks of the rubber or oleophilic type having the vehicle component for the ink pigments derived from various oleophilic materials such as aromatic and aliphatic hydrocarbons, drying oil varnishes, lacquers and solvent-type resins. An ink or printing fluid should be selected which is compatible with the coated screen member.
The "imaged" stencil can then be affixed to a duplicator or conventional frame such as wood or steel, the screen placed against a receiver member and ink pushed through the image voids and onto the receiver member by conventional means.
The following examples will serve to illustrate the invention. All parts and percentages in said examples and elsewhere in the specification and claims are by weight unless otherwise specified.
EXAMPLE I
A stencil was prepared as follows. An 80 × 80 U.S. mesh stainless steel screen was heated to above 50°C by means of a Will Scientific Company heat gun and then coated with a liquid paraffin wax, M.P. 50°-52°C (Bolar Chemical No. 1413) by dipping the screen in the melted wax. A zinc oxide photoreceptive paper (Brunning 2,000) was charged with negative corona and exposed to a silver halide positive transparency producing an electrostatographic latent image positive in sense. The charged paper was contacted by hand to the screen in the dark, and then separated. The screen was then allowed to cool to room temperature to harden the wax. In room light, a wax positive image was found on the screen consisting of voids or perforations in image configuration. The screen was then placed over a sheet of paper and an ink roller run over the screen to force ink through the voids and the image was reproduced on the paper. The printing step was repeated several times and prints of good contrast were obtained.
EXAMPLE II
The procedure of Example I was repeated but with the following exceptions. The wax in Example I is replaced with a catalyzed but uncured silicone elastomer gum [30 percent by weight poly(dimethylsiloxane) in xylene sold by Dow Corning under the designation Silastic No. 182 for use as a release agent for paper], and the screen doctored with a flexible blade. After imaging the screen is placed in an oven for approximately 2 hours at 170°C and the silicone gum hardened. Prints of good image contrast were obtained, employing the screen.
Having described the present invention with reference to these specific embodiments, it is to be understood that numerous variations may be made without departing from the spirit of the present invention and it is intended to encompass such reasonable variations or equivalents within its scope.
In stencil printing a porous-filled sheet is opened up where it is desired to make an ink flow for imaging; no ink can pass through unopened, blocked areas. Stencil printing includes both screen process printing and mimeograph duplicating. Stencil screens can be made by photoresists. For example, gelatin can be coated on a screen, sensitized by dichromate solution, dried, exposed and washed out to give an imaged screen. Ink can pass through where light has not struck. In mimeograph duplicating, imaged stencils consist essentially of ink-impervious coatings cut to expose a permeable sheet that permits ink to pass through. The stencils are wrapped around a rotating drum of a duplicator. This drum is perforated to permit ink brushed through it to pass to the stencil to image paper fed against the stencil on the drum.
In general, the processes for producing stencils are costly and time consuming. It is to this problem that this invention is directed.
DESCRIPTION OF THE INVENTION
It has now been discovered that stencils can be prepared rapidly by an inexpensive simple camera speed imaging process. More particularly, it has been found that when the voids of a screen are filled by coating with a liquid but hardenable material which is impervious to a printing fluid, not dissolvable by said printing fluid and which has a viscosity and conductivity to permit removal by electrostatic attraction, the screen can be contacted in the absence of light with a member having an electrostatographic latent image and the members separated to cause the liquid to be removed from the screen by electrostatic attraction in the charged areas of the image to provide voids in image configuration, and the liquid can be hardened in the remaining areas to form an imaged stencil master. Thus because the stencil is formed from a screen, it is not necessary to use etch solutions in order to form the image but rather the liquid can be removed from the screen to provide voids by simple electrostatic attraction. In addition, it is not necessary to form light sensitive coatings on the screen as the image can be easily and rapidly formed on a photoconductive paper such as a conventional zinc oxide master and the screen imaged by contacting the charged master with the screen and separating the members to remove the liquid from the screen in image configuration. Further, the stencils can be prepared from conventional materials and employed for both screen process printing and mimeograph duplicating. Other benefits will be apparent from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
Screens which can be employed for the printing stencil include mesh, fabric or cloth formed of any weavable material such as silk, nylon, Dacron and the like fibers as well as metal wires such, for example, as stainless steel. The screen will preferably have from 100 × 100 to 250 × 250 U.S. mesh, although larger and smaller screens can be employed.
Liquid materials suitable for filling the screen member include waxes, and a variety of natural polymerizable monomers and synthetic polymers which are of a low viscosity or can be made of a low viscosity. It is only necessary that the material be sufficiently liquid when applied to the screen such that it fills the voids, that the material can be hardened such that it is impervious to the printing fluid employed and not dissolvable by the printing fluid, and that the fluid can be removed from the screen by electrostatic attraction. Thus it can be appreciated that a large number of materials can be employed.
The liquid but hardenable material can be removed from the screen member by capillary action even if the viscosity is high when the members are contacted with the imaging member for a relatively long time. It is preferred, however, that the liquid material have a sufficiently low viscosity and resistivity that they can be quickly removed by use of an electrostatic field. Nearly all of the materials are sufficiently conductive as it is only necessary to have a resistivity of less than 10 13 ohm/cm and for the few materials which are below this conductivity, a common electrolyte such as salt can be added. Likewise, the viscosity is not a critical parameter, it only being necessary for optimum results that the liquid material and ink can be selectively removed by an electrostatic field. In order to obtain the desired viscosity, the monomers or polymers can be dissolved in a suitable solvent in which they are soluble, as is well known to those skilled in the art.
The liquid materials which can be employed to fill the screen member include materials which are hardenable at ambient temperature, or elevated temperature and which can be hardened or cured by cooling, air, or some form of radiant energy such as heat and ultraviolet light. Photo-hardenable polymers which can be cured by light are particularly preferred as it is not necessary to adjust the temperature of the screen member. The particular liquid employed will depend on the desired life of the screen member, the substrate to which it is to be adhered and other variables such as cost and availability.
Waxes can be employed which are solid at room temperature and can be made liquid at elevated temperature. Representative examples include: beeswax, carnauba wax, parafin waxes and the like. Preferred waxes are those melting below 100°C and most preferably below 60°C.
Liquid polymers which can be employed to fill the screen member include low viscosity thermoplastics, thermosetting plastics thermosetting resins and photopolymers. Exemplary of suitable thermoplastics are polyethylene, polypropylene, ethylene-vinyl acetate copolymers, propylene-modified polyethylene, acetals, acrylics, acrylonitrile-butadiene-styrene (ABS), polystyrene, cellulosics, shellac, chlorinated polyether, fluorochemicals, polyamides (nylons), polyimides, phenoxies, and vinyls. examplary of suitable thermosetting plastics are aminoplasts (urea-formaldehyde, melamine-formaldehyde), phenolics, epoxies, diphenyl oxide, polyurethanes, polyesters, diallyl phthalates, and silicones. Exemplary of thermosetting resins are vinyl, acrylic, alkyd, polyurethane, silicone, phenolic, and epoxy resins.
Polymerizable monomers can also be employed to fill the screen member. For example, monomers such as methyl, ethyl and propyl methacrylate, styrene and pivalolactone can be employed with an initiator. Initiators which can be employed to polymerize the aforesaid monomers (other than pivalolactone) include conventional materials such as for example, benzoyl peroxide, and azo-bis(isobutyronitrile). An initiator for pivalolactone is triphenyl phosphine. A number of materials can be employed and it is only necessary that the initiator or catalyst be soluble in the monomer or very finely dispersed.
Exemplary of suitable photopolymers are the cinnamic resins of polyvinyl alcohol, cellulose, starch and the epoxy resin of epichlorohydrin and 4,4'-isopropylidenediphenol. Polymethacrylate and polyamide coatings can also be employed when mixed with photosensitive polymerizable materials.
The screen member can be filled by conventional means. For example, it may be dipped in the liquid material or draw bar coated.
The thermoplastic materials and waxes which are solid at ambient temperature can be converted to a low viscosity by heat so as to render them electrostatically removable. The thermosetting polymers are liquid at ambient temperature or below their curing temperature and the photopolymers are liquid until cured such as by ultraviolet light.
If desired, a screen member can be prepared by filling the voids of the screen member at elevated temperature with a wax or thermoplastic material and then allowing the material to harden by cooling. When it is desired to image the screen member, the material can be softened by heat to fluidize the material so as to permit removal in the charged areas by electrostatic attraction.
In order to prevent the removal of the liquid material from the screen network (i.e. threads or lines of the screen) and from the interstices in the nonimaged areas because of physical contact when the screen and imaging member are contacted and separated, it may be necessary that the screen be doctored, after the voids are filled and before imaging, with a flexible doctor blade such as soft rubber, nylon or polyurethane. In this manner, the fluid is removed from the lands, and below the contact plane of the interstices so that when the screen is contacted with and separated from the image member, the liquid is removed from the screen by electrostatic attraction in the charged areas of the image and not from the noncharged areas by physical contact. Whether or not it will be required to doctor the screen depends upon the method of application of the liquid material, its consistency and affinity for the screen network.
The electrostatographic image can be formed by conventional means such as xerography. By this method the image is formed on a photoreceptor imaging member such as a zinc oxide paper master or a selenium drum. Then the electrostatographic latent image on the imaged member is contacted to the coated screen in the absence of light and the two members quickly separated. The contact and separation of the two members causes the liquid to be removed from the screen in areas corresponding to the charged areas. The liquid remaining in the noncharged areas on the screen can then be hardened by heat, air, ultraviolet light or the like, the particular treatment depending on the material employed as the liquid. Images of either positive or negative sense can be formed.
In addition to the photoreceptors mentioned, other conventional photoreceptors can be employed. Typical photoreceptors include inorganic materials such as cadmium sulfide, cadmium sulfoselenide, mercuric sulfide, lead oxide, lead sulfide, cadmium selenide and mixtures thereof dispersed in binder or as homogeneous layers. Typical organic photoreceptors include pigments such as quinacridones, carboxanilides, triazines and the like. The electrostatographic latent image can be transferred directly from a photoreceptor to the screen member or it can be first transferred to a dielectric material such as Mylar polyester and the dielectric material contacted with the screen member. Further, the electrostatographic image can be formed without the use of a photoconductor by applying a metal stencil over a dielectric member such as a polyester film and the surface charged according to the pattern in the stencil. The charged film is contacted to the screen member, separated and the fluid hardened in the nonimage areas as described above.
When it is desired to make a positive image from an image which is negative in sense, one may charge and expose a photoconductor to activating electromagnetic radiation to provide a negative image and bias (charge) the screen to a potential which is approximately the same as those areas of the image which are of the higher potential so that when the two members are contacted and separated, the nonimage areas are developed to provide a stencil with an image which is positive in sense.
Typical inks and printing equipment can be employed with the stencil master of the invention. Typical inks include inks of the rubber or oleophilic type having the vehicle component for the ink pigments derived from various oleophilic materials such as aromatic and aliphatic hydrocarbons, drying oil varnishes, lacquers and solvent-type resins. An ink or printing fluid should be selected which is compatible with the coated screen member.
The "imaged" stencil can then be affixed to a duplicator or conventional frame such as wood or steel, the screen placed against a receiver member and ink pushed through the image voids and onto the receiver member by conventional means.
The following examples will serve to illustrate the invention. All parts and percentages in said examples and elsewhere in the specification and claims are by weight unless otherwise specified.
EXAMPLE I
A stencil was prepared as follows. An 80 × 80 U.S. mesh stainless steel screen was heated to above 50°C by means of a Will Scientific Company heat gun and then coated with a liquid paraffin wax, M.P. 50°-52°C (Bolar Chemical No. 1413) by dipping the screen in the melted wax. A zinc oxide photoreceptive paper (Brunning 2,000) was charged with negative corona and exposed to a silver halide positive transparency producing an electrostatographic latent image positive in sense. The charged paper was contacted by hand to the screen in the dark, and then separated. The screen was then allowed to cool to room temperature to harden the wax. In room light, a wax positive image was found on the screen consisting of voids or perforations in image configuration. The screen was then placed over a sheet of paper and an ink roller run over the screen to force ink through the voids and the image was reproduced on the paper. The printing step was repeated several times and prints of good contrast were obtained.
EXAMPLE II
The procedure of Example I was repeated but with the following exceptions. The wax in Example I is replaced with a catalyzed but uncured silicone elastomer gum [30 percent by weight poly(dimethylsiloxane) in xylene sold by Dow Corning under the designation Silastic No. 182 for use as a release agent for paper], and the screen doctored with a flexible blade. After imaging the screen is placed in an oven for approximately 2 hours at 170°C and the silicone gum hardened. Prints of good image contrast were obtained, employing the screen.
Having described the present invention with reference to these specific embodiments, it is to be understood that numerous variations may be made without departing from the spirit of the present invention and it is intended to encompass such reasonable variations or equivalents within its scope.