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FLEXO Magazine : August 2008
TECHNOLOGIES & TECHNIQUES FIGURE 1. Stress-strain or CFD (Compressive Force Deflection) curve of foam shows how cells allow for compression till the point it begins to behave like a solid. The portion circled in red is the useful region of impression for the printer. FIGURE 2. The effect of adding a foam layer is shown on printing of a single dot. Open and Closed Case The Difference Between Various Mounting Tapes By Brett Kilhenny I t would be diffi cult to overstate the importance of foam in fl exographic printing. Foam compensates for thickness varia tions in plate and substrate, allows better control of impres- - sion force, and absorbs vibration allowing for faster press speeds while delivering higher print quality. The vast majority of fl exo printers today use foam in their mounting systems to achieve the best print results and improve effi ciency. How does foam achieve these benefi ts? The simple answer is foam is compressible. Unlike a solid, when foam is compressed, it provides a pushback force that remains essentially unchanged for a considerable amount of its thickness. A solid, on the other hand, will continually increase its pushback force from the point of fi rst contact. Foam is able to do this because of the air in its structure. As it is compressed, the supporting solid portion of the foam moves into the empty voids as shown in Figure 1. Figure 1 also contains a graph used by engineers to show how materials behave when impressed. This stress-strain curve plots the “pushback force” (stress) as the material is compressed (strain). The fl at portion of the curve is the region of impression that is useful to the printer. Here the impression level (strain) can vary, but the pressure of the plate against the substrate (stress) remains essentially the same. We can better see what this means for fl exo printing in Figure 2. The image on the left uses only a thick photopolymer plate that distorts signifi cantly when impressed resulting in considerable dot gain. The image on the right has signifi cantly less distortion because the foam layer compresses to minimize distortion while still ensuring suffi cient contact force for ink transfer. THE NEED FOR REBOUND In addition to being compressible, foam must also rebound quickly to its original shape in order to be useful for fl exo printing. In a typical wide-web application with an impression of 4mils on a combination of 20mil tape with 0.067in. plate, printing an 18in. re- peat at 1,000fpm; the foam is compressed one-fifth its thickness, 22 times per second (assuming plate and anilox impression are equal). The two most common foams in fl exography use different mechanisms to achieve this rebound force. Closed cell polyeth- ylene foams use a lightly cross-linked blend of polyethylene and 5 6 F LEXO EVA (ethylene vinyl acetate) co-polymer in which the trapped air provides a signifi cant portion of the rebound force when compressed, much like when one tries to squeeze a balloon. The other type of foam is open cell urethane that uses a cross-linked polyurethane elastomer that acts like a collection of springs, re- turning the material to its original shape after being compressed. The open cell structure allows free passage of the air within the foam and plays no role in supplying a rebound force. Both of these materials have a proven track record, but there exists much controversy as to which is better for a given application. A CLOSER LOOK It is quite remarkable that these foams can be used inter- changeably in many applications in light of how different they are from one another. Figure 3 is a SEM cross section that clearly shows the difference in their cell structures. The view on the left shows the micro cellular structure of an open cell urethane. The smaller circles within the larger cells, which look like little portholes, are cell openings between cells. This structure is called open cell. The view on the right shows the closed cell polyethyl- ene foam. The cells are larger and angular in shape. There are no openings between cells, so each cell is analogous to an individual micro-balloon. As one can imagine, because these foams use different mecha- nisms to achieve compressibility and rebound, they have very different properties. In general, the open cell urethane mounting tapes have a higher density of 20 to 30pcf (pounds per cubic foot) and rely on changes in the chemistry of the elastomer to control compressibility (whether the tape is soft or fi rm). The closed cell polyethylene tapes are not as dense as urethane tapes, typically 4-18pcf. They vary their density (i.e. how much air is entrapped) to control whether they are soft or fi rm, hence the terms “high density” for a fi rm tape and “low density” for a soft tape. Some manufacturers of polyethylene tapes have recently introduced new versions with modifi ed polyethylene blends to provide prop erties beyond what can be achieved by varying density alone. There are three properties of these materials that are signifi- cantly different and occasionally factor into the selection of one over the other. AUGUS T 20 0 8 www. f l e x o g r a p h y. o r g - BEGINNER FLEXOGRAPHER
Flexo Sustainable Fall 2008