GB2095418A - Thermal stencil process - Google Patents

Abstract

To produce a stencil from an original document without damage to the original the original is covered by an intermediate sheet, preferably a glassine paper, coated on its exposed surface with a heat-sensitive layer. The heat-sensitive layer is placed in contact with an ink-permeable layer coated onto a base sheet and by exposure to infrared the areas of the heat-sensitive layer over the original image are melted and thereby bonded to the ink- impermeable layer. The stencil is then formed by stripping the intermediate sheet from the stencil sheet, thereby removing the ink-permeable layer in the image areas. The ink-permeable layer, as described in 2,029,324A, preferably contains zinc oxide in a resin binder and is attached to a porous tissue base sheet by an adhesive layer. Preferred compositions of the heat- sensitive layer are described.

Description

SPECIFICATION Thermal stencil process The present invention relates to a thermal stencil process, that is to say a thermal process for making a stencil form an original document.

Thermal stencils have been available commercial lyforten years or more and machines for exposing the stencil master and original document to infra-red radiation while they are in contact are in common use. The process of passing the stencil master and original document through such a machine and then stripping them apart to produce the stencil is simple and rapid but the quality of reproduction from such a stencil does not match that available from other duplicating and copying processes and there is a danger of damage to the original document.

A first type of thermal stencil which is available is based on the system described in U.K. Patent Specification 1,229,016. The stencil consists of a tissue impregnated with a meltable mixture of resins and oils and mounted onto a conventional backing sheet. An uncoated tissue is attached to the front of the stencil, overlaying the impregnated tissue. The original to be copied is placed face upwards between the stencil and its backing sheet and the assembly is passed through an infra-red machine. The impregnated layer melts in the areas corresponding to the image leaving holes in the coating through which ink can pass during duplication. The stencil has the disadvantage that the thermal layer, on melting, is transferred not only to the tissue overlay but to the original.Thus the original can easily take on an oily appearance especially if the same original is used to make more than one stencil.

A second type of commercially-available thermal stencil, probably based on the method described in U.K. Patent Specification No. 1,059,553, has a thin pre-stetched film stuck to the surface of a tissue. It is perforated by the film splitting and retracting back in the image areas where it is heated. It suffers from drawbacks in that in big black areas the cutting is poor and the duplicated copies have poor infill, and also the plastic film layer of the stencil sticks to the image areas of the original and can easily damage it.

In accordance with the present invention there is provided a thermal process for making a stencil from an original document using a stencil master having a base sheet covered with an ink-impermeable layer which can be bonded to another sheet in image areas and torn away from the base sheet to form the stencil, wherein an intermediate sheet which comprises a substrate with a heat-sensitive coating is placed over the original document with the substrate acting as a protective cover in contact with the original, the stencil master is placed with its inkimpermeable layer in contact with the heat-sensitive layer of the intermediate sheet, and the assembly is exposed to infra-red radiation which passes through the stencil master and the intermediate sheet and is selectively absorbed by the dark areas of the original, the heat-sensitive layer being thereby heated in the image areas and caused to bond to the inkpermeable layer, after which the intermediate sheet is stipped from the stencil master to form the stencil.

Preferably the stencil master used in the process is that described in U.K. Patent Specification No.

2,029,324A, which is available commercially under the Trade Mark "Prestofax". The stencil master there described is intended for bonding to an electrophotographic image of the original document and thus requires a two stage process of copying the original and bonding the copy to the stencil master sheet. The process in accordance with the present invention allows a stencil to be produced direct from the original in a single stage while avoiding the risk of damage to the original which is present with the known thermal stencil sheets. Also, when the "Prestofax" stencil master is employed a better quality of reproduction is obtainable than with the known thermal stencils.

By way of example a process in accordance with the invention employing a "Prestofax" stencil master will now be described in more detail. Reference is directed to G.B. 2,029,324A for detailed description of the structure and function of the stencil master.

To image the stencil, the original is placed face uppermost between the "Prestofax" stencil and its backing sheet. The intermediate sheet is placed sensitive side uppermost over the original. The entire sandwich is then passed through a thermal machine of the type commonly used for heatsenstive copying which embodies a tungsten incandescent lamp with a colour temperature of 2500 to 3000"K, for example the machine sold under the name "Roneotherm" by Roneo Alcatel Limited.

Heat reflected in areas corresponding to print the original melts the heat-sensitive layer on the intermediate sheet. The melted material forms a bond between the stencil tissue layer and the intermediate sheet which rapidly 'sets' as the melt resolidifies.

The orignal is then removed from the stencil sandwich and the stencil is mounted on a stencil duplicator in the normal manner. The backing sheet is then removed and the intermediate sheet is pulled away from the stencil. The image areas of the stencil are retained on the intermediate sheet leaving perforations in the stencil through which the ink flows during duplication.

The substrate for the intermediate sheet must be fairly thin, infra-red transparent paper on which a coating will adhere and not be released during or after melting and resolidifying of the selected image areas. It must have good solvent hold-out so tha when it is coated the solvent that deposits the sensitive layer does not penetrate the substrate more than is necessary to give good adhesion and the sensitive layer is deposited as a thin film.

Examples of suitable substrates are glassines such as 20-40 g.s.m. Bleached Glassines maufactured by Wiggins Teape, and Vegetable Parcments and Tracing papers. The preferred material is 30 g.s.m.

Bleached Glassine.

The heat-sensitive layer coated on the substrated must be such that it melts sharply when subjected to infra-red radiation and forms a strong adhesive bond between its substrate and the stencil on melting and re-solidfying. This latter process must be rapid to permit the stencil/intermediate sandwhich to be separated as soon as the stencil has been removed from the thermal machine and put on the stencil duplicator.

The heat sensitive layer consists of two main types of ingredients. One is a resin or mixture of resins with or without added tackifiers. These ingredients provide the adhesive nature of the layer. Examples of resins used are polyamides, acrylics, polyesters, and ethylene vinyl acetates. Examples of tackifiers are Rosin and Abitol (R.T.M. Hercules Products).

The other ingredient of the layer consists of a meltable substance or mixture of substances which may be crystallisable chemicals or may be resins that improve the overall sharpness of melting of the layer. Examples of suitable substances are: Azodiisobutyronitrile Benzil Acetanilide Phenacetin Naphthanquinone Example of coated layer 90 g of Versamid 933 (RTM General Mills Chemicals Inc.) were dissolved in a mixture of 520 g of Toluene and 130 g of Industrial Methylated Spirits.

10 g of 1,4 Napthaquinone was then stirred in until a clear liqid was obtained. The solution was coated with a wire-wound rod onto 30 g.s.m. Glassine and the solvents evaporated off to give the thermally sensitive layer of 3 g.s.m. coating.

Claims (8)

1. Athermal process for making a stencil from an original document using a stencil master having a base sheet covered with an ink-impermeable layer which can be bonded to another sheet in image areas and torn away from the base sheet to form the stencil, wherein an intermediate sheet which comprises a substrate with a heat-sensitive coating is placed over the original document with the substrate acting as a protective cover in contact with the original, the stencil master is placed with its inkimpermeable layer in contact with the heat-sensitive layer of the intermediate sheet, and the assembly is exposed to infra-red radiation which passes through the stencil master and the intermediate sheet and is selectively absorbed by the dark areas of the original, the heat-sensitive layer being thereby heated in the image areas and caused to bond to the inkimpermeable layer, after which the intermediate sheet is stripped from the stencil master to form the stencil.

2. A process as claimed in claim 1 in which the base sheet of the stencil master is a porous tissue sheet and the ink-impermeable layer comprises a synthetic resin composition containing finelydispersed zinc oxide, the ink-impermeable layer being bonded to the tissue sheet by an adhesive.

3. A process as claimed in claim 2 in which the ink-impermeable layer comprises between 4 and 10 parts by weight of zinc oxide to one part of synthetic resin.

4. A process as claimed in claim 2 or 3 in which the synthetic resin composition of the inkimpermeable layer is as claimed in any of claims 4 to 8 G.B. 2,029,324A.

5. A process as claimed in any of the preceding claims in which the substrate of the intermediate sheet is a glassine.

6. A process as clamed in any of claims 1 to 4 in which the substrate of the intermediate sheet is bleached glassine.

7. A process as claimed in any of the preceding claims in which the heat-sensitive layer of the intermediate sheet is composed of an adhesive component consisting of one or more synthetic resins with or without added tackifiers and a meltable component giving a sharp melting point to the layer.

8. A process as claimed in claim 7 in wich the meltable component is a crystallisable organic compound.