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FLEXO Magazine : August 2008
TECHNOLOGIES & TECHNIQUES FIGURE 3. SEM cross-sections of open cell polyurethane (left) (100X) and closed cell polyethylene (right) (50X). Compression set resistance and long run capability. The open cell urethane has a greater resistance to compression set compared to closed cell polyethylene. This property is measured by com- pressing the foam to ½ of its original thickness for 24 hours and then releasing the pressure and re-measuring the thickness after 30 minutes, the percentage of thickness lost is the compression set. The typical values for open cell urethane are 2 to 3 percent, while closed cell polyethylene ranges from 10 to 30 percent. What does this mean for the printer? It can mean that on long runs open cell urethane will be able to provide more consistent print results whereas most closed cell polyethylene tapes will begin to lose impression and require replacement. Energy dissipation. When the print area of the plate contacts the substrate, the action can be likened to an impact. The foam layer underneath the plate will control the nature of this impact much in the same way shock absorbers control what you feel when your car’s tires “impact” the road. Extending this analogy, one can have different shock absorbers to deliver a different level of perfor- mance, depending on the preferences of the driver; fi rm shocks for a sporty ride, softer shocks for comfort. It turns out open cell poly- urethane and closed cell polyethylene handle energy dissipation very differently and this can result in differences on press. Open cell polyurethane is typically more energy absorbing than closed cell polyethylene. This difference is readily seen using the drop ball rebound test. When a ball is dropped on the open cell urethane, the ball bounces 25 to 35 percent of its original drop height. In the case of closed cell polyethylene the ball bounces 40 to 50 percent of its original drop height meaning the material is more energy refl ective or resilient. What does this mean on press? The resilience plays an important role in control of “banding” as well as overall print quality. Banding is visible web direction variation in print results, typi- cally in tonal areas that appear to be “chatter” or “gear bands.” There are many causes for it including machine vibration as well as graphics design of the plate. Many studies have demonstrated that the combined resilience of the plate and tape can infl uence the amount of observed “banding.” If this problem plagues your work it would be worthwhile to investigate changing the type of tape to determine if it can help eliminate or reduce this defect. In some instances using a more resilient tape can help, but in other situations, a more energy absorbing tape is required. It is well known that the compressibility of a cushion tape, soft or fi rm, will infl uence the dot gain and solid ink density. It turns out that the resilience of the cushion tape will also have an effect. Figure 4 below shows a plot of compressibility and resilience for a 58 F LEXO number of common cushion tapes of both the closed cell polyeth- ylene and open cell urethane types. At a given compressibility, the open cell polyurethane tapes have lower resilience than closed cell polyethylene. Interestingly, the closed cell polyethylene tapes from two different manufacturers form a nearly straight line showing the similarity in the underlying foam technology. When facing the challenge of achieving good ink coverage without excessive dot gain, changing the “shock ab- sorbers” can be one way of improving the end result. Unfortunately, at present no guidelines for selecting a given tape based on the combination of compressibility and resilience exists; since the plate (analog or digital), line speed, substrate, anilox and ink all play a role. However, the resilience of the tape is an often- overlooked factor that could make the difference between mediocre and stunning print results and should be included when troubleshooting a print problem. Stiffness and cohesive FIGURE 4. A compressibility-resilience plot of both open cell urethane and closed cell polyethylene tapes. strength. The above two properties infl uence the handling aspects of a tape. Open cell urethane tapes are less stiff than closed cell polyethylene and therefore “drape” dif- ferently during mount- ing. In general, if one is accustomed to working with closed cell polyeth- ylene tapes, some modifi- cation of how the tape is mounted will be required for open cell urethane tapes. The initial line-up of the tape to the cylinder to insure uniform contact of the tape along the cross web direction is more crucial with open cell urethane tapes. There are also differences when demounting the two tapes. Although the open cell urethane is more durable in compres- sion compared to closed cell polyethylene, it is actually weaker in tensile. This means that more steady and slow removal rate is re- quired during demounting to prevent tearing of the tape. CHOOSING A TAPE The above discussion has hopefully educated the reader that the question is not open cell versus closed cell but really a question of different polymer foam technologies that use the cells differ- ently. Both foams require the space the cells provide in order to be compressible, but the closed cell polyethylene uses the trapped air within its cells to contribute to the compressibility and resilience of the foam. The open cell polyurethane relies entirely on the poly- mer for these properties. If this article has made you curious to experiment, you should fi rst ask yourself what issues do you face: Do you have long runs, are you unable to reach high line speeds, do you struggle with banding, is it diffi cult to achieve solid ink density while minimizing dot gain, do you have problems in the mounting department? Hopefully the information above will provide some direction in your efforts to improve your print quality and pressroom effi ciencies.? ABOUT THE AUTHOR: Brett Kilhenny is R/bak business man- ager at Rogers Corp. AUGUS T 20 0 8 www. f l e x o g r a p h y. o r g
Flexo Sustainable Fall 2008