INTERGRAF International Confederation for Printing and Allied Industries

Transcription

1 INTERGRAF International Confederation for Printing and Allied Industries Screen printing Process description Screen printing is a form of stencilling where the ink is forced through the clear elements of the stencil on to the substrate to be printed. Originally known as silk screen printing because it employed silk as the fine mesh which was stretched tightly across a frame, it now uses a thin wire or nylon or polyester mesh as the stencil. The process is very versatile as it can be used to print on almost any surface or material of any size, shape or thickness, including not only all the usual printing materials but others such as metal, wood, plastic, glass, cork and fabric. Screen printing is capable of putting down a heavier ink film than any of the other processes. A variety of equipment is available for printing different types of job and lengths of run. It may be manually operated, semi-automatic, or fully automatic. Screen meshes are usually made from nylon, polyester or fine stainless steel. Screen stencils can be hand-cut, or produced photographically, or by computer-controlled stylus. The material to be printed on is positioned under the stencil screen in close contact, the frame is loaded with ink and the ink forced through the clear parts of the stencil by a squeegee. The printed material is racked separately to aid air drying or passed through a hot air dryer when higher productivity is required. Alternatively, the substrate is dried by exposure to ultra violet light. The process can produce brilliant solid colours, or lighter colours on a dark substrate. Inks can be formulated which are opaque and hide any matter underneath, or transparent to make use of the colour combinations available by halftone screen printing. Low set-up costs have always made the process attractive for short runs, but due to increased automation and process improvements, it is now a major printing process capable of very high-quality printing across a wide range of products from one colour through multiple special colours to four-colour process. Frequently produced products include posters, T- shirts, printed plastic products such as automotive decals, transfers for decorating china, printed circuits, large display boards, banners, fabric printing and cover overprints. Introduction to the recommendations Most screen printing is done with solvent-based inks. Instead of these, water-based, UV curing inks may be used. The alternative products are not fit for all of the many different substrates used in screen printing, and they cannot meet all of the many complicated demands that may be put to the products which are printed. Some examples of the demands which at present cannot be met other than with solvent-based inks: Adhesion to certain plastic substrates

2 Strong resistance against fading by exposure to UV light Strong resistance to all weather conditions Fluorescence In order to meet these special requirements, some inks still contain pigments based on toxic heavy metals such as chromium and lead. Also, although rare, some inks may contain halogenated solvents. Water-based inks still contain 5 to 15% solvents. They dry in much the same way as solventbased inks. The application of UV curing inks demands special dryers and screen qualities. The environmental implications of water-based and UV curing inks are different from those of solvent-based inks. Obviously the solvent emissions are less, but on the other hand, if not handled adequately they can give rise to water pollution. Water pollution may occur mainly during cleaning activities. See for that subject the following paragraph on cleaning of presses and ancillary equipment. There are definite trends towards avoiding the use of heavy metal pigments and halogenated solvents and towards substituting solvent-based inks by water-based or UV curing inks. Thresholds In order to stay in conformity with the VOC directive, a distinction must be made between sheet-fed and web-fed screen printing and for web-fed plants between printing on textile & cardboard and printing on other substrates. Only web-fed screen printing is in scope of the directive. Only plants with a solvent consumption above the indicated thresholds are in scope of the directive: Web-fed, textile & cardboard Large VOC installations Web-fed, other substrates Small VOC installations Large VOC installations Solvent consumption > 30 t/a t/a > 25 t/a Large and small sheet fed plants For sheet fed screen printing plants, that are not in scope of the directive, there is also a distinction made between small and large plants: Small sheet fed plants: employing < 30 production personnel Large sheet fed plants: employing > 30 production personnel A large sheetfed plant, employing 30 production personnel, working in sheet-fed screen printing, completely with solvent-based inks, may have a solvent consumption of some 15 t/a. In web-fed screen printing, such as done on textile and wall covering, the ratio between solvent consumption and personnel employed will be different. The solvent consumption per employee will be larger. Unabated situation

3 In the unabated situation all the inks used are solvent-based, and some contain halogenated substances. All these solvents evaporate and are emitted without control. The truly unabated situation still occurs, although the present use of halogenated solvent in inks in Europe is minimal. The total amount of screen printing ink sold in Europe is circa 18,000 Tons. In the printing plants, some solvent is added. In their press ready state, solvent-based inks have a solvent content of circa 60 %. If all inks were solvent-based, this would amount to some 10,000 tons. Today already a considerable amount of UV curing and water-based ink is used. Abated situation In the abated situation, no halogenated solvents are used. The solvent emissions from all large plants will be reduced considerably. From each large plant in scope of the VOC directive no more than 25 to 30%, will remain of the unabated emissions. On average, they will do better, since where a complete switchover to solvent free inks is possible, this will be preferred above keeping two different ink systems in one plant. Also from large plants that are not in scope of the VOC directive, considerable emission reduction may also be expected. Here variation between plants will be greater, due to the great variation in substrates and quality demands in each plant. Considering the fact that most of the solvents are used in installation in the scope of the VOC directive and other large plants, an overall reduction of some 2/3 of the unabated emissions seems attainable. Recommendations 1: Reduce total solvent emissions to no more than 30 or 25 % of the unabated solvent input 2: Substitute solvent-based inks where economically and technically possible Possibilities for the future 3: Substitute solvent-based inks where economically and technically possible Not recommended 4: No monoaromatic solvents in inks 5: Abate ozone emissions from UV curing inks Applicable to All installations in scope of VOC directive (i.e. web fed in excess of threshold) Large plants not in scope of VOC directive (i.e. sheet fed employing > 30) Small plants not in scope of VOC directive (i.e. employing < 30) 1: Reduce total solvent emissions to no more than 30 or 25 % of the unabated solvent input (RECOMMENDATION) There are in principal three practical methods of reducing solvent emissions from screen printing: substitution, incineration and reduction of fugitive emissions. Since the minimum investment in abatement is 200,000, all but the largest screen printer will not be able to afford that abatement method. The alternative is substitution, possibly in combination with some control over the remaining fugitive emissions.

4 Remarks This recommendation regards installations in scope of the VOC directive. Substitution allows the application of the reduction scheme. The remaining emissions need to be at a maximum 30 or 25% of the reference emission. (Small respectively large VOC installations). This point can be achieved by substituting approximately 70 or 75% of the solvent-based inks. Screen printed products are used for many different purposes and are made of many different substrates. The possibilities for emission reduction therefore vary from plant to plant. The plants should be allowed maximum flexibility in the choice of the method by which they wish to reduce emissions. It must be noted that the possibilities for substitution in screen printing are limited. It is estimated that with present technology potentially between 20 and 30% of all solvent based screen printing inks can be substituted. These figures do off course not necessarily apply to each individual plant. It must be stressed that achieving a 70 or 75% substitution will be an enormous task for most the screen printing plants in scope of the VOC directive. In most plants a rapid progress towards this goal must not be expected. Cost The cost of alternative inks is generally higher. UV inks are twice as expensive as solventbased inks. This is most relevant in screen printing, as the share of ink in the total costs in screen printing is often over 20%. Where UV curing inks are introduced, drying systems need to be altered. A saving is however incurred due to reduced downtime, increased press speed, larger output due to finer meshes and no energy consumption for the dryer when the press is not running. The savings may outweigh the additional cost. Provisions All installations in scope of VOC directive. (See: thresholds) 2: Substitute solvent-based inks where economically and technically possible (RECOMMENDATION) See previous recommendation. This recommendation is limited to the scope of the VOC directive and therefore to web-fed screen printing. It is aimed at all large screen printing plants (i.e. those employing > 30). This includes sheet-fed screen printing. In sheet-fed screen printing, the variety of substrates and quality demands much larger than in web-fed screen printing. Not only are there large differences between plants, but also within one plant. This means that it is not possible to quantify a required substitution rate that would be valid for all these plants. The possibilities for substitution need to be evaluated on a plant specific basis. Remarks The alternative ink systems are still under development. Introducing them requires experiments, management time, technical expertise and tests for customer approval. It must be

5 ensured that a large enough management and supporting staff is available to carry the substitution through successfully. This is only available in large screen printing plants. It must be noted that the possibilities for substitution in screen printing are limited. It is estimated that with present technology potentially between 20 and 30% of all solvent based screen printing inks can be substituted. These figures do off course not necessarily apply to each individual plant. It must be stressed that achieving a 70 or 75% substitution will be an enormous task for most the screen printing plants in scope of the VOC directive. In most plants a rapid progress towards this goal must not be expected. Cost See previous recommendation. See also above under remarks: the switch over will cost a considerable amount of management time and machine time for tests. This is an additional reason to limit the recommendation to large plants. Provisions The recommendation is limited to plants employing > 30. The recommendation is limited to wherever this is technically and economically possible. In this particular case this not only means that a non solvent alternative must be available, but that the amount of solvent-based ink which may be substituted is such that it warrants all the tangible costs. For small plants see below. 3: Substitute solvent-based inks where economically and technically possible (POSSIBILITY FOR THE FUTURE) Once the substitutes for solvent-based inks have been developed further and more practical experience exists within the industry, it may be possible to widen their application to small plants also. 4: No monoaromatic solvents in inks (NOT RECOMMENDED) Aromatic solvents include solvents like toluene and xylene. Although these are nowadays rarely, used in screen printing, there is no environmental need to prohibit their application. Toluene and xylene solvents are widely applied elsewhere in industry, without any other precaution than adhering to the OEL. There is no need to substitute these solvents any faster than any other solvent. 5: Abate ozone emissions from UV curing inks (NOT RECOMMENDED) Curing of UV inks takes place under a strong UV lamp. This UV lamp causes the occurrence of small amounts of ozone. This ozone may pose a health & safety problem, if not extracted and vented outside the factory. The amount of ozone produced by the UV lamps is so small that it has no environmental significance. Abatement is not necessary.

6 FURTHER INFORMATION THE INFORMATION ABOVE WAS PROVIDED BY INTERGRAF IN JUNE IT IS BASED UPON PRINTING AND THE ENVIRONMENT, THE AWARD WINNING DOCUMENT PROVIDING GUIDANCE ON BEST AVAILABLE TECHNIQUES IN PRINTING INDUSTRIES. THIS DOCUMENT COVERS ALL THE EXISTING PRINTING PROCESSES AND CONSIDERS OVER 200 DIFFERENT CANDIDATES FOR BEST AVAILABLE TECHNIQUES. IT IS AVAILABLE IN ENGLISH, SPANISH AND POLISH. CONTACT INTERGRAF OR NATIONAL PRINTING FEDERATIONS FOR FURTHER INFORMATION. Intergraf Ms.AnneMarie DeNoose ADDRESSES 18 Square Marie Louise (bte , B1040 Brussels, Belgium Phone: , Fax: Maetis Consultancy BV Mr.Paul W.Verspoor MBA Postbox 1644, 3600 BP Maarssen, The Netherlands Phone: , Fax: