SCREEN PRINTING PROCESS AND SCREEN PRINTING FORME SUITABLE FOR USE THEREIN

Abstract: A screen printing process, in which a screen printing forme is made using a photopolymerizable material in the form of a sheet, preferably 0.2-0.7 mm thick, which before or after exposure and development is firmly secured to the screen. The thickness of the sheet, the nature of the screen and the nature of the printing ink are selected so that the resulting printed pattern, after drying, has a maximum thickness of at least 0.25 mm.

Description: Screen printing process and screen printing forme suitable for use therein. This invention relates to a screen printing process, in which a screen printing forme is made using a photo- polymerizable material. Oth ar Zeman, in an article in "Der Siebdruck", Vol. 27, January 1981, pp. 24 ff., describes the manufacture of sensitized screen printing materials, and indicated in particular how a thicker layer of sensitizing material can be obtained, in order that prints with a larger thickness may be made. For chromate- or diazo-sensitized materials, the impregnated screen may be further laminated to a thickness of at most 0.3 mm above the screen level. When a larger thickness (0.3-2 mm) is required, use is made of a liquid photo- polymerizable mass, which is solidified by the image-wise exposure, and at the same time attaches itself to the screen material, i.e., is anchored in the gauze or fabric. The two techniques described above are very cumber¬ some, require great skill to be correctly carried out, and have the inherent disadvantage that the screens, which are relatively expensive, can only be used once. Dutch patent application 7607224 discloses making braille script by screen printing, in which a normal screen printing forme is used, i.e. one that has not been made thicker, and a printing ink is used which contains a swelling agent. The printed pattern applied, or paste deposited, is then expanded by a heat treatment. This method has the disadvantages that the degree of swelling, i.e. the thickness of the printed pattern, cannot be accurately controlled, that its implementation requires a large amount of energy, and that many materials are not resistant to the high temperature of the heat treatment (180 C) . Moreover, as a result of the formation of bubbles, the surface of the dry printed pattern is not smooth and even rather rough, which is unpleasant to "read". It is an object of the present invention to provide a screen printing process which avoids the above and other drawbacks and- disadvantages. To this effect the invention provides a screen printing process in which a screen printing forme is made using a photopolymerizable material, characterized in that the photo- OMPI polymerizable material is used in the form of a sheet which before or after exposure and development is firmly secured to the screen, the thickness of the sheet, the nature of the screen and the nature of the printing ink being selected so that the resulting printed pattern, after drying, has a maximum thickness of at least 0.25 mm. The photopolymerizable sheet material can very easily be pressed down on the screen, or attached to the screen doctored with liquid photopolymerizable material, while the expensive screen can afterwards be easily and fully cleaned for re-use. Furthermore, the thickness of the printed pattern can be accurately controlled, drying does not require a large amount of energy and not a high temperature either, and the dry printed pattern consists of a hard, undeformable material with a smooth surface. Photopolymerizable materials which are solid-at normal temperatures, and can be provided in the form of a flexible sheet are known per se. Suitable materials of this kind are photo¬ sensitive compositions of the nylon type, soluble in water, in dilute acid or in alcohols. Such a photosensitive resin composition comprises, for example, as essential components a polymer containing a basic nitrogen atom in the main chain or in a side chain, a photopolymerizable unsaturated monomer, and a photosensitizer. For detailed information about such a photosensitive composition, reference is made to TJS patent 4,145,222. Photopolymerizable sheet materials suitable for use in accordance with the present invention are commercially available in various thicknesses from about 0.2 to about 1.2 mm under the names of Toyobo Printight (made by Toyobo Company, Ltd., Japan) and Miraclon and Rigilon (made by Tokyo Yoko Ono, Japan) . The photopolymerizable sheet, to which a protective peel-off cover foil is applied on one side, is fixedly connected on the other side to a polyester or metal backing, by means of an adhesive layer which may also have an anti- halo effect. This fixed connection with a backing is of importance, because the material is intended for making printing plates, in particular relief printing plates. For the purposes of the present invention, it is sufficient for the fixed backing in the above-described commercial products to be made temporary or of the peel-off type, which can be effected in a simple manner by just omitting the adhesive layer. According to the invention, there is preferably used a photopolyjierizable sheet 0.2-0.7 mm thick, as generally speaking, this gives the best results in producing the contemplated relatively thick printed pattern. The sheet thickness to be selected in a specific case is dictated in part by the nature, and in particular the thickness, of the printed . pattern, but also depends on the thickness of the gauze of the screen. In fact, a thin gauze comprising a large number of threads per running centimeter, to produce an equally thick printed pattern with the same printing ink, must be combined with a thicker photopolymerizable sheet than a thick gauze which contains a relatively small number of threads per running cm. Various procedures are possible for making the screen printing forme according to the invention. According to a first technique, at choice, the cover foil or the backing is removed in products as described above, and the photopolymerizable sheet is exposed through a positive original. Any suitable light source (point light or tubular lamps) radiating in the UV-range may be used for the exposure, such as a high-pressure or low-pressure mercury vapour lamp, a metal halide lamp, a xenon lamp, or a carbon-arc lamp. Care should be taken that this exposure is not too intensive, and in any case less than required to effect complete photopolymerization. This is of interest for maintaining sufficient adhesiveness of the photo¬ polymerizable sheet with a view to its subsequent adherence to the screen, and also to limit the effect of the inevitable scattering outside the actual exposure area to the extent that the areas thus exposed to scattering can still be washed away during the developing step. The exposure which in a specific case will give optimum results can be readily determined and controlled by one skilled in the art by proper selection of the light source to ' be used and the adjustment of the exposure distance and exposure time. After exposure, the forme is developed, which can be carried out in an efficient manner by means of a powerful hot-water sprayer, with the temperature of the water being preferably at least 50 C, and more preferably 60-80 C. It is of importance that the entire surface of the sheet should be treated with the water jets as uniformly as possible to ensure complete development. Generally speaking, development will take approximately 3-15 minutes, depending on conditions. The sheet which, as a result of the development, has a soft and slightly sticky feel, is then pressed against the screen for it to adhere firmly to the screen threads. This operation may be carried out, for example, in a vacuum frame. Subsequently, the covered screen is dried, for example, in a drying stand or with a stream of hot air at approximately 30-60 C, and subjected to after- exposure with TV radiation, as described before, to complete the photopolymerization or cure the photopolymerizable material. Finally, the backing or cover foil still present is stripped off, whereafter the screen printing forme is ready for use. According to a second technique for making the screen printing forme, the screen is first doctored with photopolymerizable material in the liquid form, as for example marketed by the above- mentioned Toyobo firm. Subsequently, the screen thus pre-treated is pressed against the free surface of a photopolymerizable sheet, for example by means of a squeegee, which sheet is on its other side provided with a strippable backing or cover foil as described above. The resulting laminate is then dried for some time, for example, 20-40 min. at a relatively low temperature, preferably no higher than 25 C, to ensure firm adherence, whereafter the backing or cover foil still present is stripped off and the photopolymerizable sheet is exposed, developed, dried and subjected to after-exposure in the manner discussed in more detail hereinbefore in conjunction with the first technique for making the screen printing form to be used according to the present invention. A great many screen materials may be used for carrying out the present screen printing process, such as, among other materials, nylon, polyester, metallized polyester and steel. It is of importance for the screen to contain not too few and also not too many threads per running cm. Fewer than 15 threads per running cm is generally insufficient for a firm adherence of the developed photo¬ polymerizable sheet, especially in areas with little photopolymerized material only, whereas with more than 73 threads per running cm its permeability to the printing ink to be used is insufficient for the screen to be completely filled in one or a few squeegee passes only. Preferably, screensare used containing 24-51 threads per running cm. For printing, the screen ready for printing is OMP disposed so that its coated side or relief side faces the article to be printed, and spaced some distance from it. This distance, which may be referred to as the gap, varies depending on the thickness of the screen printing forme, the printing ink used, the nature and desired thickness of the printed pattern, and the type of article being printed. Commonly, the gap is between approximately 2 and 6 mm The printing operation proper can be effected in the manner conventional for screen printing processes, namely, by applying the printing ink to the screen and pressing it through the screen meshes and depositing it on the article being printed using, for example, a squeegee of rubber or synthetic plastics material. As stated before, the present invention contemplates producing a relatively thick printed pattern, namely, one in which the maximum thickness of the individual characters, after drying, is at least 0.25 mm. In this connection the thickness of the photo¬ polymerizable sheet, in combination with the nature of the screen, in particular the thickness of the gauze, are of importance, as described hereinbefore. A further important item is the nature of the printing ink. As a matter of fact, it should have a high solid content, and is allowed to exhibit, at most, only a slight degree of secondary flow. The solid content of the printing ink used is preferably at least 80%. Suitable printing inks of this type are the known per se IR-curable inks. Particularly suitable for use according to the present invention are the (also known per se) UV-curable printing inks having a solid content of substantially 100%, and which immediately after printing can be dried throughout by UV-irradiation in.a short period of tim , for example in 10 seconds to 1 minute,, whereby secondary flow and loss of height of the printed image are minimized. The fast drying or curing of the printing inks last-mentioned is based on a UV-initiated polymerization, which need not be discussed in more detail herein, as both the principle and countless practical implementations thereof are described extensively in the literature. UV-curable printing inks are marketed, for example, by Advance-excello, Chicago, United States of America, under the name of Solex. The present screen printing process has numerous applications, such as for braille printing, decorative relief printing, • and printing for electronic purposes. Depending on the particular application, the most widely divergent flat materials may be printed, OM such as paper, synthetic plastics material ,metal, textile, glass, ceramics, etc. and all sorts of additives may be incorporated in the printing ink to suit requirements, such as colorants, pigments, electrically conducting or insulating substances, magnetizable particles, etc. When the present process is used, for example, for braille printing, it may be useful to print in colour, in order to give the advantage of colour discrimination to those still having some eyesight. Also, according to the present invention, braille can be printed on both sides of the paper, because, unlike classical braille, no deformation of material takes place. When no further prints have to be made, the screen can be easily liberated from all the material present thereon for it to be re-used. For this purpose, the screen printing form can be treated with a suitable solvent for, in particular, the photo- polymerized material present. Suitable solvents are ethanol, bleaching water, fairly concentrated formic acid or acetic acid, or combinations thereof. Owing to the action of the solvent the photopolymerized material begins to swell and becomes soft, where¬ after the screen may be sprayed clean for example with a high- pressure sprayer. Subsequently, the screen may be degreased, for example, by means of a soda solution or the like, and then be dried, whereafter it may be used for performing a next screen printing cycle. The invention is illustrated in and by the following examples. Example I To provide a sheet-shaped or laminar photo¬ polymerizable material, the starting point was a sheet of Toyobo Printight type EF-70, size A3 (Toyobo Co.,. Ltd., Japan), which was modified in that the anti-halo adhesive layer had been omitted between, the 0.55 mm thick, water-washable photopolymerizable layer and the polyester backing thereof. The protective cover foil on the other side of the photopolymerizable layer was removed, and subsequently this layer was exposed in a screen printing vacuum copying frame through a positive braille original with a carbon arc lamp of 60 A, 220 V, 16.7 k at a distance of 80 cm. for 75 sec. The material was then developed by spraying the exposed layer uniformly and vigorously with hot water having temperature of 60-80 C for 10 min. Excess water was carefully removed with a cloth, whereafter the rather soft developed layer, whose surface was somewhat sticky as a result of the development, was contacted with a screen printing frame spanned with a polyester gauze of 40 threads per running cm, and was firmly secured to the screen threads by applying vigorous suction (90% of absolute vacuum) in a screen printing vacuum copying frame for 5 min. Subsequently the coated screen printing frame was dried for 20 min. in a drying stand at 30-40 C, and subjected to after-exposure for a further 5 min. with a carbon arc lamp and under the same conditions as described above, but without the original and without vacuum, in order to complete the photopolymerization. Finally the polyester backing was removed and the screen printing forme was ready for use. The screen printing forme was then placed in a screen printer with its coated side facing downwards and so as to leave a gap of 4 mm between it and a sheet of paper to be printed. Printing was effected using a UV-curable printing ink produced by mixing Solex HUG-UV-9119 in a 75/25 ratio with Solex HUB 9050 halftone base (both being products of Advance-excello, Chicago, U.S.A.) . The printing ink was applied to the screen printing forme and pressed through the meshes and deposited on the sheet of paper to be printed by means of a squeegee of synthetic plastics material. Immediately thereafter the sheet thus printed was held under a 700 W high-pressure mercury vapour lamp (Philips type HPL) , placed at a distance of 30 cm for 30 sec. to cure the printed pattern. In this way an excellent braille print was produced with raised marks in the form of smooth, non-compressible or otherwise deformable semispheres having a maximum thickness of 0.3 mm. 200 prints of excellent quality were made with the - screen printing forme in the manner described above, and thereafter all ink was removed from the forme,, and the latter was immersed in a methanol bath for 2 min. , whereafter all the material still present on the screen printing chase was removed using a high-pressure water sprayer. The chase was then treated with a scouring and degreasing agent,- and was then ready for re-use. Example II A screen printing chase spanned with polyester gauze, as described in Example I, was coated with liquid photopolymerizable material of Toyobo Co, Ltd., subsequently contacted with the photo¬ polymerizable layer of a modified plate - also as described in Example I - the protective cover foil of which had been removed, and finally pressed down by means of a squeegee. The resulting o assembly was dried at 22-25 C for 30 min. , whereafter the polyester backing was removed. The photopolymerizable layer present on the chase, and very firmly secured to it, was then exposed, developed, dried and subjected to after-exposure as described in Example I, with the understanding that the exposure time was not 75 sec, but was selected between 90 and 150 sec. , depending on the desired degree of polymerization and the desired limitation of scattered radiation. The screen printing forme was then ready for use. Printing was carried out exactly as described in Example I, using the same printing ink, and equivalent printing results were obtained'. Finally the screen printing chase was cleaned for re-use in the same way as described in Example ϊ„

Claims: C L A I M S 1. A screen printing process, in which a screen printing forme is made using a photopolymerizable material, characterized in that the photopolymerizable material is used in the form of a sheet which before or after exposure and development is firmly secured to the screen, and the thickness of the sheet, the nature of the screen and the nature of the printing ink are selected so that the resulting printed pattern, after drying, has a maximum thickness of at least 0.25 mm. 2. A screen printing process according to claim 1, characterized by using a photopolymerizable sheet 0.2-0.7 mm thick. 3. A screen printing process according to claim 1, characterized by using for the photopolymerizable sheet a nylon-type photopolymerizable material soluble in water, dilute acid, or in alcohol. 4. A screen printing process according to claim 3, characterized in that the photopolymerizable sheet is exposed to an extent less than required to effect complete photopolymerization, and is subsequently developed with water of a temperature of at least 50 C, and preferably 60-80°C. 5. A screen printing process according to claim 4, characterized in that the exposed and developed photopolymerizable sheet is subjected to after-exposure on the screen to complete photo¬ polymerization. ' 6. A screen printing process according to claim 1, characterized by using a screen whose fabric has 15-73, and preferably 24-51 threads per running cm. 7. A screen printing process according to claim 1, characterized in that printing is effected using a high-solid printing ink curable by UV-irradiation. 8. A screen printing process according to claim 1, characterized in that, after use, the screen is freed from all material present thereon for said screen to be re-used. 9. A screen printing process according to claim 1, characterized in that it is used for braille printing, decorative relief printing, or printing for electronic purposes. 10. A screen printing forme suitable for use in carrying out the screen printing process as claimed in claim 1. OMPI

Inventors: VAN ROEKEL DAAN, VAN ROEKEL, DAAN
Applicants: ROEKEL DAAN VAN, VAN ROEKEL, DAAN
Application year: 1982
Main classification: G03F7
IPC codes: B41M1, B41M3, B41N1, G03F7