GB2101041A - Improved thermal duplicating stencil and its use - Google Patents

Abstract

The invention provides a duplicating stencil sheet assembly for thermal imaging and use in mimeograph duplicating with the production of high quality copies, the said assembly comprising a thermal stencil sheet 2 and a porous tissue overlay sheet 1, the latter being designed not only to receive melted impregnating material during the imaging step but also to remain in position during the duplicating operation between the inking screen of the duplicating machine and the imaged stencil sheet so as to regulate in an advantageous manner the passage of ink through the stencil. As compared with previously used porous tissue overlay sheets, the tissue overlay sheet of the present invention must be strengthened to resist the mechanical strains of duplicating and the duplicating ink. <IMAGE>

Description

SPECIFICATION Improved thermal duplicating stencil and its use This invention relates to thermal duplicating stencils and their use.

Thermal duplicating stencils customarily comprise a sheet of highly porous tissue, for example of the kind known as Yoshino tissue, impregnated with a heat-sensitive ink-impervious composition. In use, a heat-absorbent original to be copied is placed in contact with the stencil sheet, and the combination is exposed to infra-red radiation. Absorption of the latter by the image areas of the original causes the adjacent areas of the heat-sensitive impregnation of the stencil tissue to melt, thereby forming a stencil which reproduces the original. It is customary to mount the unimaged stencil sheet on a sheet of thicker paper called a backing sheet, which is used to provide a degree of mechanical support for the stencil sheet until the latter is placed on the duplicating machine.It is also customary to provide, on the other side of the stencil sheet from the backing sheet, a sheet of highly absorbent tissue, which may be, and usually is, of the same kind as the tissue from which the stencil sheet itself is made, but unimpregnated, so that, during the imaging process, the melted areas of the impregnation of the stencil sheet are taken up by the sheet of porous tissue in much the same way as ink is taken up by blotting paper. It will be appreciated that use of such a sheet helps to prevent the melted composition from soiling the original which is being copied, and prevents the composition remaining in the stencil sheet when the source of heat is removed, which might cause imperfections in the copies obtained.

Thermal stencils are convenient to use because they can be imaged very rapidly using only simple equipment. However, they have not enjoyed very wide-spread use in fields where quality of reproduction is a consideration, since the quality of copies obtained has often been rather low.

Analysis of this problem has shown that an important factor behind the production of poor quality copies using thermally imaged duplicating stencils is the fact that the imaged stencil passes too much ink during the duplicating process. As a result, too much ink is deposited on the sheet placed to receive the image, with resultant feathering and even smudging of the copy.

Moreover, there is a marked tendency for "set-off" to occur, i.e. for ink to be transferred from a freshly duplicated copy onto the back of the next succeeding copy during the duplicating process. Set-off is particularly unacceptable where it is desired to reproduce material on both sides of the copy paper, which is desirable in the interests of economy and minimising the amount of paper used.

It is not practical to improve the quality of the duplicated copies obtained using a thermal stencil by reducing the porosity of the tissue used in making the stencil sheet in order to reduce the amount of ink transferred during the duplicating process. This is because the low porosity paper which would be required is necessarily uneven in texture, and the inevitable fibre clumps which occur in such tissue effectively act to produce localised small areas which will not pass ink during the duplicating process. Thus, attempts to improve the quality of copies by using tissue of lower porosity fail, and the copies obtained are on the whole of even poorer quality.

It has now been discovered that the flow of ink through an imaged thermal duplicating stencil can be satisfactorily controlled so as to improve the quality of the copies obtained, and in particular to reduce the occurrence of set-off, if the imaged thermal stencil is placed on the duplicating machine without removing the porous tissue overlay sheet, provided that the latter has first been strengthened so that it can withstand the mechanical stresses of the duplicating process and prolonged contact with the duplicating ink. While the known porous tissue overlay sheets cannot be used in the present invention, it is a simple matter to strengthen them adequately so that they can be so used.

The present invention accordingly provides a duplicating stencil sheet assembly for thermal imaging comprising a thermal stencil sheet and a porous tissue overlay sheet, which during imaging is in contact with the stencil sheet to receive melted impregnating material therefrom and during duplicating is (without change of its position in relation to the imaged stencil sheet) between the imaged stencil sheet and the inking screen of the duplicating machine, and which is strong enough to resist the mechanical strains of duplicating and the duplicating ink.

The invention also provides a method of duplicating which comprises exposing an' assembly as aforesaid to infra-red radiation in contact with a heat-absorbent original to be copied on the side of the stencil sheet away from the porous tissue overlay sheet, removing the original, placing the combination of imaged thermal stencil sheet and porous tissue overlay sheet on a duplicating machine with the porous tissue overlay sheet adjacent to the inking screen thereof, and printing copies through the said stencil.

Except in the construction and manner of use of the porous tissue overlay sheet, the thermal stencils of the present invention are the same as known thermal stencils, for example those described in United States Patents Nos. 3694244 and 37041 55. They thus preferably comprise a backing sheet to assist in mechanical handling of the stencil sheet prior to and during imaging and while the duplicating machine is being prepared for use. The thermal stencil sheet itself is made of suitably impregnated highly porous stencil tissue, both the tissue itself and the impregnating composition being of known kind.

The porous tissue overlay sheet is a sheet of highly porous tissue containing usually 5 to 25% and preferably about 8% voids which has been treated, for example by impregnation with a small amount of a synthetic resin, so as to confer on it the necessary strength characteristics.

Not only does the strengthened tissue have to withstand the mechanical strains of remaining on the duplicating machine while the stencil is being used, but it also has to withstand the duplicating ink itself. The latter is an emulsion comprising an aqueous phase and an oily organic phase. The tissue must be strengthened so as to be capable of resisting both phases. This may be achieved by impregnation with a small amount of a synthetic resin which is resistant to water and the organic phase of the ink, in an amount insufficient substantially to reduce the porosity of the tissue. Suitable resins include polystyrene, polymethylmethacrylate, polyvinylbutyral and styrene/butadiene copolymers. These may be applied to the tissue in solution in an appropriate organic solvent such as toluene, acetone, or methylethyl ketone at a concentration of, for example, 0.5 to 2% by weight.The optimum concentration of the polymer solution and the optimum coating weight on the tissue may easily be found by routine experiment.

The tissue used for the porous tissue overlay sheet is preferably the same kind of tissue as that used in the thermal stencil sheet itself, that is to say Yoshino tissue, or more usually a machine-made equivalent thereof, commonly called stencil tissue. This tissue is rather less open than the tissue used in the thermal stencil sheet itself and is preferably impregnated with high molecular weight polystyrene in a coating weight not greater than 1 gram per square metre but adequate to confer the necessary strength and ink-resistant characteristics. The polystyrene may be applied as a 1.5% by weight solution in toluene.

The treated tissue is assembled with the thermal stencil sheet in the same way as in known stencil sheet assemblies comprising a thermal stencil sheet and a porous tissue overlay sheet, except that the porous tissue overlay sheet of the invention should be rather more securely attached to ther thermal stencil sheet (usually along one edge only thereof) as it is not removed after imaging and must remain in position during the duplication process.

The manner of imaging and using thermal stencils in accordance with the invention basically the same as that for known thermal stencils except that both the imaged stencil sheet and the strengthened porous tissue overlay sheet remain in place during the operation of the duplicating machine. It is surprising that the residue of the impregnating composition from the stencil sheet, which has been melted during the imaging process and taken up by the tissue overlay, does not interfere with the passage of ink during the production of duplicated copies. However, any interference with the passage of ink which takes place is either negligible or beneficial.

To secure the best possible quality of copies using the new thermal duplicating stencil assemblies, it is necessary that, during the imaging process, the original to be copied is accurately in contact with the thermal stencil sheet over the whole of its surface. With some types of original, it is difficult to ensure that this is so. Moreover, some originals, e.g. those having non-black images, absorb heat inadequately to give good quality copies. These difficulties may be avoided by preparing first from the original to be copied a sub-master using a normal electrostatic copying machine capable of producing a black image from the original to be copied. The sub-master is then used to prepare the imaged thermal stencil.

The following Example illustrates the invention Example Stencil tissue weighing 10-12g./m2 is coated with one of the following compositions to a total weight (including the tissue itself) of 36-44g./m2.

Parts by weight (1) Cellulose acetate-butyrate EAB 500-1 13.3 parts Staybelite ester 5 6.7 parts Mobilsol L 24 parts Hercolube A 9.3 parts Hercolyn D 6.7 parts Antioxidant 2.1 parts Monastral Green pigment paste (l.C.l) (1:1 in oleyl alcohol) 0.2 parts Methylated Spirit 74 Overproof 33 Parts Toluene 124 parts Ethyl acetate 89 parts or Parts by weight (2) Cellulose acetate-butyrate EAB 500-1 15 parts Ethoxylated oleyl alcohol 14.9 parts Cumar R9 or Piccoumaron 410EH 16.7 parts Mobilsol L 28.6 parts Antioxidant 2.4 parts Monastral green pigment paste 0.2 parts Toluene 154 parts Isopropyl alcohol 42 parts Ethyl acetate 11 3 parts Cellulose acetate-butyrate EAB-500-1 is a low viscosity grade manufactured by Eastman Kodak having a butyrate content of 48%.

Mobilsol L is a refined naphthenic petroleum oil from Spcony-Mobil Oil Co. Texaco spindle oil may be used in its place.

Staybelite ester 5 is a glycerol ester of hydrogenated rosin manufactured by Hercules Powder Co.

Hercolube A is a pentaerythritol ester of a saturated fatty acid manufactured by Hercules Powder Co. Hercolyn D is a methyl ester of hydrogenated rosin acids manufactured by Hercules Powder Co.

The ethoxylated oleyl alcohol contains 1-2 ethylene oxide residues per oleyl residue.

Cumar R9 and Piccoumaron 410EH are coumarone/indene resins of Neville Chemical Co.

The words "Staybelite", "Mobilsol", "Hercolyn" and "Monastral" are registered Trade Marks.

After evaporation of the solvents, the stencil sheets impregnated with one or other of the aforesaid compositions are mounted coated side down on a backing sheet of kraft paper as used in known thermal stencils. The porous top sheet having the polystyrene impregnation as described above is then attached across the top edge only of the stencil sheet. (The 'coated side' is that side to which the impregnating composition is first applied).

The accompanying drawings show diagrammatically a stencil in accordance with the invention and the way it is used.

In Figure 1, a kraft paper backing sheet 3 has a top fold 4 enclosing the end of the stencil sheet 2. The pretreated porous top sheet 1 is attached to the top fold 4 along the line 6-6'.

After the stencil sheet has been thermally imaged, the assembly is attached to the duplicator and inked. After the initial inking the backing sheet 3 is torn off at the perforations indicated at 5.

Figure 2 shows diagrammatically the arrangement of the sheets during imaging. The document to be copied 7 is placed between the stencil sheet 2 and the backing sheet 3 with the material to be copied in contact with the former. The top sheet 1 remains in place while the imaging is brought about by exposure of the assembly to heat.

After the exposure to heat with consequent imaging of the stencil sheet and removal of the original, the assembly is placed on a duplicating machine as shown diagrammatically in Fig. 3.

The top sheet 1 remains in contact with the inking screen of the duplicating machine and itself supports the imaged stencil sheet 2. The backing sheet has been removed by tearing along the perforations 5 to leave only the top fold 4 holding together the stencil sheet and the top sheet.

The duplicator is then used in the usual way.

Claims (9)

1. A duplicating stencil sheet assembly for thermal imaging comprising a thermal stencil sheet and a porous tissue overlay sheet which during imaging is in contact with the stencil sheet to receive melted impregnating material therefrom, and during duplicating is (without change of its position in relation to the imaged stencil sheet) between the imaged stencil sheet and the inking screen of the duplicating machine, and which is strong enought to resist the mechanical strains of duplicating and the duplicating ink.

2. A duplicating stencil sheet assembly as claimed in Claim 1 which also comprises a backing sheet to assist in mechanical handling of the stencil sheet.

3. A duplicating stencil sheet assembly as claimed in Claim 1 or 2 wherein the porous tissue overlay sheet is a sheet of highly porous tissue containing 5 to 25% voids which has been impregnated with a synthetic resin so as to confer on it the necessary strength characteristics.

4. A duplicating stencil sheet assembly as claimed in Claim 3 wherein the said highly porous tissue has been impregnated with polystyrene, polymethylmethacrylate, polyvinylbutyral or a styrene/butadiene copolymer.

5. A duplicating stencil sheet assembly as claimed in Claim 4 wherein the porous tissue overlay sheet is a sheet of highly porous tissue containing about 8% voids which has been impregnated with high molecular weight polystyrene in a coating weight not greater than 1 g./m2 but adequate to confer the necessary strength and ink resistance characteristics.

6. A duplicating stencil sheet assembly for thermal imaging as claimed in Claim 1 substantially as hereinbefore with reference to the accompanying drawings.

7. A duplicating stencil sheet assembly for thermal imaging as claimed in Claim 1 substantially as described in the foregoing Example.

8. A method of thermal duplicating which comprises exposing a duplicating stencil sheet assembly as claimed in any one of the preceding claims to infrared radiation in contact with a heat-absorbent original to be copied on the side of the stencil sheet away from the porous tissue overlay sheet so as to image the said stencil sheet, removing the original, placing the combination of imaged thermal stencil sheet and porous tissue overlay sheet on a duplicating machine with the porous tissue overlay sheet adjacent to the inking screen thereof, and printing copies through the said stencil.

9. Duplicated copies produced by the method of Claim 8.