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printreg, January, 2001
Re: Information on the difference between printing processes


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From:Gary Jones(gjonesprinting@aol.com)
Date:Monday, January 08, 2001 12:26 PM


Pat: The quick answer to your question regarding MACT applicability is that it does not apply to lithographic printing operations. The title of the rule has lead to some confusion on both the part of industry and permitting authorities as to the applicability of the rule. However, EPA's fact sheet on the final rule and sections 63.820 and 63.821 state that the rule is to only cover wide web flexo, publication rotogravure, and packaging gravure operations. Lithographic printing is very different from flexographic and rotogravure printing. The difference lies in both the inks and application methods, principally the image carrier. In lithography, the inks are very thick and paste-like with a consistency close to peanut butter. In flexography and rotogravure, the inks are very fluid, much like water. In lithography the image carrier or plate is very different as the image and nonimage area are virtually in the same geographic plane. In flexography, the image area is raised relative to the nonimage area and with rotogravure, the image area is recessed relative to the nonimage area. Lithography and flexography use plates, with the flexo plates being made of rubber or other flexible polymers. Litho plates tend to be rigid and can be made of aluminum, paper, or plastic. Rotogravure uses a chrome plated cylinder as an image carrier. In terms of process descriptions, these definitions are contained in the New Jersey DEP's State of the Art Guide for Graphic Arts: Lithographic printing is defined as follows: A planographic printing system where the image and nonimage areas are chemically differentiated; the image area is oil receptive and the nonimage areas are chemically differentiated; the image area is oil receptive and nonimage area is water receptive. Ink film from the lithographic plate to an intermediary surface (blanket), which, in turn, transfers the ink film to the substrate. Fountain solution is applied to maintain the hydrophilic properties of the nonimage area. Ink drying is divided into heatset and nonheatset. Fountain solution is defined as follows: A mixture of water and other volatile and nonvolatile chemicals and additives that maintains the quality of the printing plate and reduces the surface tension of the water so that it spreads easily across the printing plate surface. The fountain solution wets the nonimage area so that the ink is maintained within the image areas. Non-volatile additives include mineral salts and hydrophilic gums. Alcohol and alcohol substitutes, including isopropyl alcohol, glycol ethers, and ethylene glycol, are the most common VOC additives used to reduce the surface tension of the fountain solution. Heatset is defined as follows: A lithographic web printing process where heat is used to evaporate ink oils from the printing ink. Heatset dryers (typically hot air) are used to deliver the heat to the printed web. Non-heatset is defined as follows: A lithographic printing process where the printing inks are set without the use of heat. Traditional non-heatset inks set and dry by absorption and/or oxidation of the ink oils. For the purposes of this rule, ultraviolet-cured and electronbeam-cured inks are considered non-heatset, although radiant energy is required to cure these inks. Flexographic printing is defined as follows: A printing system utilizing a flexible rubber or elastomeric image carrier in which the image area is raised relative to the nonimage area. The image is transferred to the substrate through first applying ink to a smooth roller that in turn rolls the ink onto the raised pattern of a rubber or elastomeric pad fastened around a second roller, which then rolls the ink onto the substrate. Rotogravure printing is defined as follows: A printing system utilizing a chrome plated cylinder where the image area is recessed relative to the nonimage area. Images are transferred onto a substrate through first applying ink to a cylinder into the surface of which small, shallow cells have been etched forming a pattern, then wiping the lands between the cells free of ink with a doctor blade, and finally rolling the substrate over the cylinder so that the surface of the substrate is pressed into the cells, transferring the ink to the substrate. Screen printing is defined as follows: A printing system where the printing ink passes through a web or fabric to which a refined form of stencil has been applied. The stencil openings determine the form and dimensions of the imprint. Letterpress printing is defined as follows: A printing system in which the image area is raised relative to nonimage area and the ink is transferred to the substrate directly from the image surface. For more information, there are two EPA documents that contain printing process descriptions. There is a link on PNEAC's web page at http://www.pneac.org/sheets/all/index.cfm to EPA's Printing Industry and Use Cluster Profile that has great process descriptions. Also, there is another publication called Profile of the Printing Industry (EPA 310-R-95-014) that has process descriptions. It can be obtained from EPA at 800/490-9198. I am not aware that this document is available on-line. In addition, there is a widely used publication entitled Pocket Pal. It provides a general overview of the printing industry and contains basic description of the process. It is published by the International Paper Company and is available from the Graphic Arts Technical Foundation. GATF also offers other publications on the different printing processes. GATF can be contacted at: www.gatf.org or www.gain.net. 412/741-6860



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