Three-layer stencil assembly having plastic overlay sheet

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate prior art stencils capable of being prepared by means of a typewriter only.

FIG. 3 shows a prior art thermostencil known as the “Weber stencil.”

FIG. 4 illustrates a second prior art thermostencil known as 75 the “Riso stencil.”

THREE-LAYER STENCIL ASSEMBLY HAVING PLASTIC OVERLAY SHEET

FIELD OF THE INVENTION

The present invention relates generally to improvements in the mimeograph stencil art and more particularly to a new and improved stencil assembly wherein the stencil is capable of being prepared by means of a thermocopier and by means of a conventional typewriter.

DESCRIPTION OF THE PRIOR ART

One of the most critical problems concerning designers of mimeograph stencils has been the problem of designing a stencil which can be prepared by means of a thermocopier or by conventional typing. This problem is overcome by the present invention. To completely understand the instant invention it is necessary to outline the prior art stencils that are available and to describe their differences.

Broadly, there are four types of mimeograph stencils: One is prepared by the use of a typewriter or a blunt pointed instrument referred to as a stylus. A second can be prepared by means of a photographic system, whereby a sheet of emulsion coated tissue is exposed photographically and the blacks on the emulsion are eliminated chemically leaving exposed apertures. A third type is prepared by the use of an electronic scanning device which burns tiny apertures into a plastic sheet. The fourth and newest type, referred to as a thermal stencil, is prepared with a thermocopier. Referring now to the conventional typing stencils, there is shown in FIG. 1 a typical prior art typing stencil which consists of two sheets, a first backing sheet which is made of a heavy although flexible paper much like that from which postcards are made, and second sheet which is the stencil layer and is made by coating or impregnating a sheet of fine fibered tissue paper with a film of highly plasticized material, such as nitrocellulose on both sides or it can even be coated instead with wax. Fig. 2 shows a modification of the conventional typing stencil wherein a clear plastic overlay is placed on top of the stencil layer sheet. Openings which would represent alphabetical letters, numerals, or drawn lines are made in this waxlike coating by striking this coating or if an overlay is present, by striking this overlay with the type face of a typewriter key. In either case, ., no part of the waxlike coating is actually removed. It is merely “pushed” to the outer edges of the holes or aperture made by the typewriter or the stylus. With the advent of electric typewriters however, the conventional stencil has suffered from what is commonly referred to as “chop-outs.” This means that the center of the “0,” “a,” “e,” etc. can be actually cut out and thereby destroy the perfection of the stencil. There are actually two reasons for this: first, because the force of the strike by the typewriter not only pushes aside the waxlike coating, but also cuts the fibers of the tissue; and second, because most plasticizers used for coating the stencil sheet have a pH rating on the acid side and, after a time and exposure to oxygen, the tissue fibers become brittle and thus break very easily. While the conventional clear plastic overlay shown in FIG. 2 primarily accomplishes the function of preventing “chop-outs it does not overcome the problem existing in “pushing” the wax material to the side of the letters thereby causing a broader aperture. Therefore, the current clear plastic overlaid stencils prepared by typewriter do not yield an image as clear and sharp as desired without “chopouts.”

The other type of stencil to which this invention relates is the thermal stencil. Shown in FIG. 3 is the most popular type of thermal stencil known in the trade as the “Weber stencil.” This stencil comprises a conventional backing sheet of flexible oil resistant paper. Attached to this backing sheet is a plasticized plastic coated tissue stencil sheet. Attached to this layer is a tissue layer which in thermal stencil making acts as a receptor sheet. The original sheet which is to be copied is placed face up between the backing sheet and the plasticized

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FIG. 5 is a diagrammatic view illustrating a preferred embodiment of the invention when utilized with a thermostencil.

FIG. 6 is a diagrammatic view illustrating a second embodiment of the invention when utilized with a conventional typewriter stencil.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 5, which illustrates a preferred embodiment, a stencil assembly 11 capable of being prepared by either thermocopying or by conventional typewriting. The stencil assembly is a three-layer stencil having a backing layer 13, a stencil layer 14, and a coated overlay layer 17 and in some respects is similar to the stencil shown in FIG. 3 and described as the prior art. Backing sheet 13 is conventional and is made from a sheet of heavy flexible paper. Fastened to backing sheet 13 along a line 20 is the stencil layer 14 consisting of a tissue layer 15 coated with a plastic substance 16. Attached to this sandwich along the line 20 is the ²⁰overlay sheet 17 consisting of a tissue 18 coated with a film of elastomeric plastic 19 on its uppermost side. While any elastomeric plastic such as polyvinyl chloride or polypropylene may be used, polyvinyl chloride is presently preferred. Layer 17 gives the stencil the capability of both ²³being a thermostencil and a conventional typewriter stencil. This layer acts as both a cushion sheet and an absorbing receptor layer. When a typewriter is utilized for preparing the stencil, this layer prevents “chop-outs due to the plastic surface 19 and prevents “pushing” of the stencil plastic coating ³θ material due to tissue layer 18 actually absorbing the plastic material from stencil layer 14 thereby providing a clean aperture.

A further result of providing the cushioning sheet with the plastic film 19 is evident when this same stencil assembly is ³³used as a thermostencil; a more uniform coating 16 and consequently, a cleaner and sharper aperture in stencil sheet 14 is achieved. One reason for this occurrence is the fact that a great concentration of heat is transferred to the stencil sheet at the points of contact between the characters on the original ⁴⁰and the coated stencil layer when the plastic film is present than with the, previously referred to, prior art stencils.

Now referring to FIG. 6, which constitutes an alternative embodiment, there is shown a typewriter stencil assembly 21 wherein an elastomeric plastic coated overlay 17 is placed on ⁴³a conventional typing stencil such as that shown in FIG. 1. The stencil assembly comprises a backing layer 13 connected to a stencil sheet 23 which is comprised of either a wax or cellulose coating 25 placed upon a tissue layer 24, the stencil sheet and backing sheet being fastened together along a line 20 in a con- ⁵⁰ventional manner. Also attached along line 20 is the coated overlay 17 which, as described above in connection with FIG. 5, is made up of a tissue layer 18 and an elastomeric plastic coating 19 laminated thereto. As described with reference to the prior art, the conventional typing stencils shown in FIG. 1 ⁵⁵suffer from the defect of having “chop-outs” and from a “pushing” effect of the wax stencil coating when struck by a typewriter key. By utilizing the overlay layer 17 these effects are eliminated. The elastomeric plastic coating 19 acts as a cushioning layer for the typewriter keys thereby preventing ⁶⁰“chop-outs” while tissue layer 18 of the laminated structure absorbs the wax or cellulose coating from stencil sheet 23 thereby preventing “pushing” of the stencil coating material. This results in a clean aperture. This absorbent feature is completely lacking in any of the prior art stencils. ⁶³

While the results obtained from utilizing this elastomeric plastic laminated overlay appear deceptively simple, it is to be emphasized that the use of this overlay layer now provides a stencil assembly which is capable of both preparation by conventional typing means or by means of a thermocopier. It further provides an absorbent tissue layer to remove any excess material from the stencil layer and gives a cushioning ef5 feet which prevents “chop-outs.” When the stencil sheet is prepared with a thermocopier this elastomeric plastic coated overlay provides a higher melting temperature than previous overlays thereby absorbing more material from the stencil sheet. Even utilizing this layer with a conventional typing sten10 oil as shown in FIG. 6 an improved result is obtained by providing a cushioning layer as an absorbing layer thereby preventing “chop-outs” and “pushing” effects which have previously not been accomplished. It should be understood, of course, that the foregoing disclosure relates to only preferred 15 embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.