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FLEXO Magazine : March 2011
Technologies & Techniques position. Rolls are rewound from different locations within the master reel of paper and can have subtle differences from one end of the reel to the other. Running the same positions together helps your press operator make adjust- ments to achieve optimum quality in running and printing. • Store paper properly. Proper control of both paper stor- age areas and pressrooms can substantially improve ef- ficiency in production. Allow paper rolls time to acclimate to the temperatures at which the product will be running in the pressroom; avoid running rolls that are too cold. Controlling pressroom humidity will help minimize static or curling. Ideally, humidity should be controlled in the range of 45-55 percent RH. • Adjust for new papers. Printing on different paper sub- strates can require adjustments to press equipment and materials such as plates, mounting tape, anilox rolls, inks and surfactants. Coated substrates generally require high- er volume anilox rolls. Additionally, the printer will likely need to adjust ink viscosity and chamber pressures as well as consider adding surfactants. Finally, higher density sheets may require the printer to use a less compressible mounting tape to match paper characteristics better. • Use paper resources. When you identify opportunities to improve press performance with your substrate (uptime, running speeds, print quality, etc.) contact your supplier for technical assistance. Have the following information on hand to help you maximize results from your supplier: A clear description of the issues or opportunity. Samples or pictures that illustrate what you’d like to achieve. A determination about whether the issue is continuous or potentially varies within a roll or between rolls. Specific roll identification numbers. Contact your paper supplier to understand its roll numbering system. Paper, along with other flexographic substrates, has inher- ent variability because of its manufacturing process and number of suppliers. By applying some basic paper knowl- edge and tapping into the expertise of paper suppliers, a flexo printer can dramatically enhance the print performance of a particular project. n About the Authors: Randy Neil and Norm Doucette are Technical Service Representatives for Twin Rivers Paper Company. Each brings over 25 years of paper and printing experience to their role of collaborating with packaging print- ers and converters to ensure optimal usage of the company ’s products. Twin Rivers Paper Company is an integrated specialty paper manufacturer of lightweight paper for the packaging, label and publishing markets with operations in Maine and New Brunswick. KeY PAPeR PROPeRTIeS Structural ProPertieS Dimensional stability. The degree to which a paper will maintain its size and shape when subjected to changes in moisture content or relative humidity. Porosity. The compactness of its fibers determines the degree to which a paper will allow the permeation of air, gas or liquid, indicating absorbency. Caliper. The thickness of a sheet of paper under specific conditions. These characteristics are important to understand and account for in the printing process because they affect print registration, ink laydown and absorption and run- nability on the press. Surface ProPertieS Brightness. The refractive properties of the surface. Bright papers give cleaner, crisper color and better con- trast with black ink. Brightness is not necessarily related to color or whiteness. As technology advances, paper manu- facturers continue to increase the brightness of papers in all quality levels, including free-sheet and groundwood. Opacity. The percentage of light passage through a sheet of paper. Basis weight, brightness, type of fibers, fill- ers, coatings and formation influence opacity. Smoothness. The surface flatness of the sheet, which affects ink receptivity and transfer. Fillers, coatings, super- calendaring and sizing can enhance the smoothness. Brightness and opacity affect color contrast and show-through for applications that are printed on both sides or for applications where shadowing of the pack- aged item is not desired. Smoothness affects how the ink sits on the surface, the crispness of lines and the image registration. chemical ProPertieS Moisture content. The paper’s moisture. Paper will change from its original state when exposed to differ- ing levels of moisture, resulting in tight or wavy edges that affect printability and runnability. Moisture content affects the dimensional stability and tensile properties of the sheet, which in turn affects print registration and runnability on press. FTA FIRST recommends a moisture content of 5.5 percent (+/-1.5 percent). Sizing. Additives applied to improve printing quali- ties or resistance to liquids. Sizing that is used to impart short-term moisture repellency may affect ink absorption and drying time. 46 FLeXO march 2011 www.flexography.org FLX_March11.indd 46 3/18/11 1:33 PM