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FLEXO Magazine : February 2009
TECHNOLOGIES & TECHNIQUES moist sheet will require more ink and multiple press adjust- ments to hold register. Excessive moisture can result in wrinkles and wavy edges making downstream converting difficult. Papers with low moisture content are more susceptible to web breaks on press. Sizing. Sizing refers to chemicals added to improve the paper's end-use performance. Surface sizing involves applying a light film of starch (or other material) to one or both sides of the web with a size press. Sizing reduces surface lint- ing and increases the resistance of paper to absorbing liquids such as wa- ter or ink. It improves ink holdout by slowing the rate of ink absorption into the fiber structure of the sheet. This reduces the risk of wicking, feather- ing, chalking, and print density loss. TEST METHODS The International Organization of Standardization (ISO) has developed many widely accepted substrate test methods. In addition, the Technical Association of the Pulp and Paper Industry (TAPPI) has assembled some of the most widely used pa- per and paperboard test methods. Several of these are referenced in FIRST 4.0, and are summarized in Table 1 (see page 27). The Appendix in FIRST 4.0 also contains ISO and T APPI contact information. Specifications and tolerances vary widely since many are customer (buyer) driven. Additionally, there can be large differences within sub- strate groups, depending on gauge, basis weight, grade, supplier, etc. Specifications and tolerances for customer and supplier are to be mutually established, based on print quality requirements and process capabilities. The specifications defined within FIRST 4.0 should be viewed as generally acceptable limits; tighter specifications may be required for specific applications. LAMINATION AND COLOR MATCHING Color matching on press, when printing a design that will be laminated in a downstream operation, is particularly challeng- ing. A reverse-printed web, that is part of a lamination, will have a very different color off-press (pre-lamination) than it will after final lamination. FIRST 4.0 recommends a couple of ways to simulate the final lamination in order to achieve the desired color match on press. - The first is to laminate the reverse-printed web to a similar secondary substrate press-side using a tabletop laminator (de- signed to laminate paper documents and posters). This can be done by first cutting the image to be laminated from the press tear sheet. Next, place the image on a non-sealable carrier sheet ink side up (paper will work as a carrier in most cases). Then cut a piece of secondary substrate (simulate lamination) and place it on top of the printed sample. Finally, cover this with a non-sealable carrier sheet and pass through the heated lamination device. With proper se- lection of the secondary substrate, press-side lamination should be a close match to the final product and can be used for color matching procedures. The second option is to laminate the printed press characterization target on production equipment. By measuring the printed character- ization target both laminated and unlaminated, the resulting color shift can be quantified and predicted. The data from the laminated press characterization can then be used to show the expected color shift on a digital proof. Printers/convert- ers should be aware that dark, wet products, such as soups or meats, can dramatically alter the appearance of the graphics compared to the graph- ics mounted as a sample on white board stock. FLUTE SELECTION When choosing the right flute profile for a package, the strength requirements of the package and the desired graphic quality must be considered. There is an inverse rela- tionship between board strength and print quality. As flute height increas- es (N to F to E to B to C) board strength also increases; however, print quality decreases. Thinner flutes, such as E, N, and F, tend to minimize the appearance of wash boarding or flute lines, resulting in improved appearance of graphics. When a package requires both increased structural strength and high-quality graphics, a combination flute is often utilized. For example, an E-B double wall, in which a printed E-flute board is glued to the outside of a B-flute board, accomplishes both objectives. . FEBRUARY 2009 www.f I exog ra p hy.o rg FLEXO
Sustainable Winter 2009