CA1114232A - Duplicating stencil - Google Patents

Abstract

Abstract of the Disclosure A duplicating stencil is produced by placing an electro-statically-produced image on a substrate in contact with an ink-impervious surface layer of a stencil blank, bonding the image to the surface layer by the application of heat and/or pressure and stripping the substrate from the stencil blank to remove the surface layer in the image area. The surface layer may comprise a synthetic resin composition containing a finely dispersed pigment and is attached to the porous base tissue of the stencil blank by an adhesive.
Bonding of the image to the surface layer and possibly also fixing of the image to the substrate in the same operation can be effected by means of infra-red heating.

Description

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DVPLICATING STENCIL
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This invention relates to the preparat on of stencils for.use on a stencil duplicator for reproduction purposes.
The principle of stencil duplicating is a .1ong establish ~' ed art which invoives the use of an impermeable stencil which is perforated or cut in areas corresponding to an image required to be duplicate~ so that when the stencil is used ~` on a duplicator machine ink passes through the holes in the s~encil and produces duplicate images on the copy pape--empl oyed .
. 10 There are at the present time two commercial systems suitable for the non-manual production of stencils. The f:irst process involves the electrical cutting of a carbon-~ aded layer on a porous stencil.sheet. A spark discharge .~ cut~ing-head is synchronised with an optlcal head whicn views the original and directs the cutting-head to cut the ~: stencil, point by ~oint. This system produces good results but is relatively slow, requiring a period of 5 to 15 minutes to complete,. and since the process involves burning off the carbon loaded regions it is dirty and produces an unpleasant odour.

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The second process is referred to as "thermal imaging"
and involves placing a thin plas-tic coated porous stencil over the face of the original document to be copied, the stencil and copy being then subject to exposure by a lamp. ~he temperature rise of the coating is highest in black image areas and the coat-ing there is arranged to melt so as to produce porous regions corresponding to the image of the original and thus the stencil can be used on a duplicating machi.ne as in the other process described to produce duplicate copies. This latter process has shortcomings in that the r~solution is poor and it is blind to many colours of the original, including some blacks. The process is very quick, the exposure time being of the order of 10 seconds, but it requires practice and skill to determine the length of exposure. There is also a risX of damage to the original.
According to the present invention there is provided stencil sheet for the production of a stencil from an electro-photographic image by bonding the image to the stencil sheet and then tearing away parts of the stencil sheet adhering to the image, the stencil sheet comprising a porous base material, an ink-impervious layer comprising a synthetic resin composition contain-ing.finely-dispersed zinc oxide, and an adhesive layer bonding the ink-impervious layer to the porous base material r the adhesive layer serving to selctively release the regions of the ink-impervious' layer bonded to the image.

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It will be appreciated that in order to get best results with the process it is necessary to have the correct relationship between the strength of the bond securing the surface layer to the base material of the stencil sheet and the strength of the bond formed between the electro-static image material and the surface layer. The latter is determined partly by the composition of the image material and the surface layer and partly by the conditions of pressure, temperature and time in the fusing stage which may comprise the application of heat, pressure, or a combination of both. The electrostatic image material, usually called "toner", is commonly a dry powder which is applied to the electrostatic latent image and subsequently fixed in place by heat or pressure. The preferred toner for use in the present process is a heat-fusible material which may in some cases be fixed by cold pressure instead of by heating but which can be fused by heating to form a bond with the surface layer of the stencil sheet. The heat-fusible toner may be of a known type comprising a mixture of carbon, iron oxide, and a synthetic resin, the latter being the fusible component.
Naturally, better results can be achieved if the composition of the ink-impervious surface layer of the stencil sheet is chosen to match the particular toner used in forming the electrostatic image. It is also important that this surface layer breaks cleanly, at the edges of the image areas and the loading of a synthetic resin composition with a powdered zinc oxide aids the formation of a clean break which will give sharp definition to the finished stencil.

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The fusing stage in which the electrostatic image is bonded to the surface layer oP the stencil sheet may be subsequent to the fixing of the image on the substrate or may be combined with the fixing of the toner powder to the substrate in a single step.
The invention will now be described in more detail with the aid of examples illustrated in the accompanying diagrammatic drawings, in which:-Figure 1 shows schematically the introduction of a stencil into : a conventional electrostatic copying machine which is modified to allow passage of the stencil through the fusing stage of the machine to fuse the image simultaneously to the substrate and to the surface layer of the stencil;
Figure 2 is a cross-section of the form of stencil blank which can be employed;
Figure 3 shows diagrammatically the separation of the stencil from the image-bearing substrate, and Figure 4 shows schematically an alternative method in accordance with the invention for producing a stencil.
In the embodiment illustrated in Figures 1 to 3 a stencil sheet or blank 9 comprises a base sheet 10 of porous tissue to which is bonded, by means of an adhesive film 11, an ink-impermeable surface layer 12. The thicknesses shown in Figure 2 ::,~ ",,, ~ ""~ :,"~ ~, ,, , ", ~ ~ :

are not, of course, to scale ~nd in ~ig~ 3 the adhesive film 11 has not been shown bec~use it is of very small thickness.
As shown diagrammatically in Figure 1 an electrostatic copier 13 is modified to allow t~e introduction of the stencil 10 into the machine ~efore the copy paper 14 has had the regions 15 of toner powder fused thereto. The means for introducing the stenci] into the electrostatic copier must be such that the coated ink-impermeable layer 12 is facing the side of copy paper 14 which carries the toner material so that once the stencil has been placed in position a set of rollers or other pick-up means feeds the copy 14 and the stencil 10 simultaneously through a pair o rollers 16 which apply heat and/or pressure to fuse the toner region 15 both to the copy paper 14 and to the surface of layer 12 of stencil 10.
The electrostatic copier whilst beirlg otherwise of standard design wou]d thus have an additional facili~y for producing stencils. Stencil blanks could, for example, be stGred inside or Gutside the machine in a cartridge storage facility.
The copy paper 14 and stencil 10 emerge from a slot, not shown, in the end of the copier 13 ~nd as illustrated dia-grammatically in Figure 3 separat;on of the copy anc stencil ~ by pulling apart produces perforations in the impermeable ; surface layer 12. These perforations correspond to the regions of toner on the electrostatic copy and there is thus ` 25 produced quickly and cleanly a stencil which when used on a duplicating machine produces sharp images having a very fine ~` resolution. The stencil is no less sensitive to colours than the electrostatic copier and the development or perforation energy is produced by the operator in peeling apart the sheets.

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No solvents are required and the dirt and odour o previous solutions are avoided.
In the embodiment shown in Figure 4 the stencil ls applied to a copy wherein the toner image has already been fused. The two sheets are placed together as befo~e and fed through a further separate fuser unit 17 between two rollers 18 such that the two sheets are bonded or fused together as in the first embodiment. Separation of the two sheets, as before, produces a stencil for use in a duplicating machine. The fusing stage in both embodiments is one which is appropriate to the toner material and non-ink permeable layer and may use heat or pressure or a combination of both heat and pressure applied by means of heated rollers.
As has been described with reference to ~igure 2 the stencil blank comprises a sheet of porous tissue, an ink-impermeable surface layer, and an adhesive film securing the surface layer to the tissue. The tissue can be a conventional stencil tissue such as Yoshino Type 602 Standard stencil tissue.
The ink-impervious layer comprises a suspension of a finely divided zinc oxide in a resin binder. The ratio of zinc oxide to binder can vary from 3:1 to 12:1. A range of resins can be used which includes acrylics, polyurethanes, poly~inylchlorides, cellulose esters, vinyl acetates, etc. Mixtures of different resins may be used to produce a layer which bonds firmly to the toner image of the electrostatically '. ' ' : .` ' .' . ' ' ' produced master copy~ The resin constitu~es the ink-imp~rvious film. It also acts as an adh~sive to bind the zlnc oxide and to form a bond with the toner image. The zinc oxide assists the bonding of the toner and interrupts the continuity of the ~esin film allowing the layer to break ra~her ~han stretch during the separation of the bonded sheets thus pxoducing the per-foration mechanism. The suspension of zinc oxide in resins is prepared by milling e.g. ball-milling, for a long time so that a small particle size is obtained which is preferred as this tendls to improve sharpness of the perforated stencil.
A dye such as Rose Bengal, Crystal Violet, Methylene Blue, Malachite Green, etc. is included in the milled suspension to improve the ; visual appearance of the perforated stencil. The prepared suspension is coated onto a casting paper or a release paper to give a dry coating weight of 20 - 35 gsm. The degree of release is selected so that the cast layer remains bonded to the paper and can only be delaminated once adhered to the tissue.
The function of the adhesive is to laminate the ink-impervious layer to the tissue but it must selectively release the regions of the layer that have been bonded to the electrostatically produced image on 2Q the substrate. Thus the bond strength of the adhesive must be sufficient to provide good lamination to the tissue allowing multiple copies to be taken on the duplicator without stencil damage but must be sufficiently weak to be overcome by the bond between the layer and the image on the substrate. Various types of :` ~'t~

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-adhesive re~ins can be used such as polyvinylacetates, cellulose deri~atives, acyrlic emulsions, etc. The bond foxmed between the tissue and the layer depends on the type of adhesive selected but this may be a dry-bond, pressure or heat-sensitive bond. The adhesive is applied to the ink-impervious layer, which had been previously coated onto the release paper, and the tissue is laminated to the adhesive layer in its wet or dry state depending on the type of adhes-ive employed. The quantiti of adhesive applied is that which provides the correct bond strength when tested.
The above laminate forms the stencil blank which may now be ~tripped from the ~elease paper or retained on the .elease paper depending on th~ partic~lar stencil format required.
15The electrostatic image used with the stencil blank ; ir the process of the inver.tion may be produced by ~irect or indirec~ electro-photography. The imaging proce-ses, which are well-known, involve producing a latent electro-static image on a substrate and then develcping the latent image with a toner powder and fixing to produce a visible, stable, image. Various types of toner are used, and ~or the present process a dry heat-fixed toner with a melting point in the range of 80 to 150~ is preferred. For example a one component, magnetic heat-fix toner can be applied to the latent image by means of a magnetic roller.
The fusing stage in which the electrostatic image is bonded to the surface layer of the stencil blank is prefer-- ably carried out using infra-red heating because in this way ~ .

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radiation ls preferentially absorbed by the bl~ck aleas of the electrostatic master copy thus providing maximum heat in the image areas where bonding is required.
The following are more detailed examples of the process in accordance with the invent:ion:~
Example I
The stencil sheet was manufactured as follows^-A finely-divided suspension of pigment in birder resin wa3 prepared by taking 200 parts by weight zinc oxide (Durham 10~, Irom Durham Cher,licals), 66 parts by weight of 50% solution of an acrylic resin in xylene (QR451 from Rohm & Hass), 100 parts by weight toluene, mixing and ball-mixing for 16 hours. The suspension was coated onto casting paper (Multithane 700 from Wiggins Teape) to a dry coating weight of 24 g/m2. The dry cast layer was ovet-coated with an aqueous, pressure-sensiti~vTe adhesive layer (Revertex A312 from Harco Chemicals) and the water dried off. Yoshiilo tissue was applied by a laminating xoller providing sl~ficient pressure to give firm adhesion.
A copy of the origina~ was taken on a Roneo Vickers Type DB6 electrostatic copier which uses a zinc oxide co~)y paper and a one component, magnetic toner powder. The toner used was a mixture of carbon black,iron oxide as a magnetic pig-ment, and a thermoplastic acrylic resin Synocryl 4003 2~ available from Cray Valley Products. The resin had a melting range of 95 to 130C. The image was pre-fixed by cold pressure to the copy paper.

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- 10~ 2~2 The ima~ed sheet was placed in colltact with the sur~ace layer of the stencil blank and the combination passed through an external fusiny unit containing a 500 watt infra-red source in a pyrex glass cylinder with the imaged side of the sheet towards the source.
The bonded composite was mounted on a Roneo Vickers Ty~e 870 Diplicator and the copy paper peeled back.
The stencil thus produces gave satisfactory duplicating performance. The duplicated copies had gocd definition and the stencil produces long runs.
Example II
As in ~xample I, except that the surface layer of the stencil blank comprised 240 parts by weight zinc oxide (Durham 100), 50 parts by weight 30~ solution of polyure-thane in toluene/Xylene/Isopropyl alcohol ~Witcote 344from Witco Chemicals), 75 parts hy weight 20% solution of cellulose ester in Toluene/Industrial Methylated Spirits ` ~C~B 381-0.5, from Eastman Kodak), 35 parts by weight 1%
solution of crystal violet in Industrial Meihylated Spirits - 2~ and 130 parts by wei~ht toluene.
Ex~mple lII
.~s in Example I except that the adhesive used to bond the surface layer to the Yoshino tissue was an aqueous suspension of a polyvinyl acetate (Monarch D.12~5 from Monarch Adhesives). The adhesive was applied to the cast layer and ihe tissue laminated to the wet adhesive. The composite was then dried.

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Example IV
As in ~xample I except that the milled suspension is coated onto a release paper (Quicklease 30/104 from Jointinel.
The laminated stencil blank is supplied on the release paper which also acts as a backing sheet to assist the hand--ling of the stencil.
Example V
As in Example I except that the copy of the original used to bond to the surface layer of the stencil was made on a U-Bix MkI plain paper cop~er, the toner being pre-fixed by heated rollers.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A stencil sheet for the production of a stencil from an electro-photographic image by bonding the image to the stencil sheet and then tearing away parts of the stencil sheet adhering to the image, the stencil sheet comprising a porous base material, an ink-impervious layer comprising a synthetic resin composition contain-ing finely-dispersed zinc oxide, and an adhesive layer bonding the ink impervious layer to the porous base material sheet, the adhesive layer serving to selectively release the regions of the ink-imperv-ious layer bonded to the image.

2. A stencil sheet as claimed in claim 1 in which the ratio of zinc oxide to synthetic resin composition is between 3 to 1 and 12 to 1 by weight.

3. A method of making a stencil sheet as claimed in claim 1 or 2 in which the ink-impermeable layer is formed on a release surface, adhesive is applied as a layer to the ink-impermeable layer and the base material is laminated to the adhesive layer by heat and/or pressure.