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FLEXO Magazine : March 2011
Technologies & Techniques have been anticipated. One must be careful to determine the impact each change has on other printing criteria (i.e . adhe- sion, retained solvents, blocking, etc.). Any change in the ink, for whatever reason, requires a recheck on the ink substrate compatibility. Printing processes are somewhat interactive; a change in one part of the process can be absorbed by another part of the process or be accommodated by a corresponding change in yet another part of the process. For example, in flexo the plate durometer and mounting tape can be adjusted separately or in combination to achieve a desired effect. The same is true regarding inks and substrates. There are a number of substrate characteristics that affect ink and the selection of specific ink chemistries. SUBSTRATE-RELATED ISSUES Slip agents / anti-blocking substances. Many films, par- ticularly polyethylene (PE), have slip agents compounded into them, or anti-blocking substances added to the surface. Slip agents migrate to the film surface to modify the co-efficient of friction (COF) of the film. They are generally waxy or greasy, which can create a barrier at the ink / substrate interface that prevents the ink from making intimate contact with the film surface and can result in adhesion problems and print defects. (See Figure 1.) Anti-blocking substances are sometimes dusty and can in- terfere with ink adhesion. In addition to creating the challenge of the ink having to work through these materials to get to the substrate surface, these materials may also show inconsisten- cies from roll to roll or lot to lot. These additives are soluble at the elevated extrusion tem- peratures, so they are initially uniformly distributed through the film. However, they function in use by being insoluble in the bulk of the film so that they slowly migrate to the surface to modify COF. The amount of material that migrates is depen- dent on the ambient temperature. Some inks (e.g., polyamides) also contain slip additives. The additives are more soluble in the ink than the film, and the ink creates an attraction for the slip agent. Inks can scav- enge a marginal dosage of additives, which can soften the ink and result in an increased tendency to block; while also changing the COF of the film. The Solutions • Try corona treatment of the film on press. This will typi- cally “burn off ” the excess slip agent and allow the ink to wet the substrate surface and anchor to the film. Without treatment, an ink with an aggressive solvent mixture would be required so that it will “bite” through the surface contamination and anchor to the film. The ability of in-line corona treatments to remove excess slip compounds from the surface will vary with the speed and watt density. Some cases will require lower press speeds to allow a high enough treatment level to address this issue. • Check lot-to-lot variations to ensure as much consistency as possible. If there is a large amount of variability, con- tact the substrate supplier and initiate corrective action. • In some cases variability can be overcome by a more robust ink system which can operate on a wider variety of surface conditions. One caveat: These ink systems may increase ink costs even as they drive down total costs. • Printers may not often be in a position to dictate substrate choice, or they may not feel empowered to do so; but choosing a consistent substrate is key. • Match ink selection vs. slip levels in the film to avoid scav- enging or seasonal variations. Modify the slip additives in the ink so that the slip agents in the film are less attracted to the ink surface. Porosity. All uncoated paper surfaces have some level of porosity which allows the ink to partially absorb beyond the surface into the underlying paper mass. But some fiber-based substrates may have a very porous surface character. This generates a sponge-like surface that will absorb the ink rather than allow it to remain on the surface while drying. A highly porous substrate may cause the ink to look weak or penetrate to the non-print side. It will also reduce gloss. Higher porosity may also allow UV inks to deeply penetrate the surface, which could inhibit the complete curing of the UV inks. (See Figures 2a, 2b and 2c.) The Solutions • Speed up the drying rate of the ink so that it will begin to dry before it can be absorbed. • Choose an ink system specially formulated for rough surfaces which is optimized for their ink hold out. • Increase the ink film thickness to flooding the surface and com- pensate for the ink that is absorbed, keeping in mind that this will increase ink use (and cost), while decreasing print fidelity. Caution: This is not a good solu- THE FUTURE IS ROUND Until now, if you wanted the superior print quality and high press speed of continuous-print flexo sleeves, you had to accept the long lead times associated with custom-made products manufactured in Europe. Faced with a fast turnaround project? Your options were limited. Not anymore. DuPont is opening its first North American manufacturing site for Cyrel® round sleeves. Finally, you will be able to enjoy the benefits of Cyrel® round sleeves—gravure-like image quality, near-perfect register, improved tonal range, excellent ink laydown, high press speed and more— all with improved lead times. Whether you’re interested in solvent-process continuous print blanks or our new Cyrel® FAST round system, your choice has never been clearer. And, the results have never been faster. At DuPont Packaging Graphics, we create print technologies—innovations that move the flexographic industry forward. Copyright © 2011 DuPont. All rights reserved. The DuPont Oval, DuPontTM, The miracles of scienceTM and Cyrel® are trademarks or registered trademarks of E.I. DuPont de Nemours and Company or its affiliates. DuPont Packaging Graphics “Advancing Flexography” www.cyrel.com/na DPG-FLEXO-ADS.indd 1 12/7/10 11:00 PM FIgURE 1. slip agent which has migrated to the film surface. surface pits and irregularities. FIgURE 2A. normal coated paper. FIgURE 2B. Coated paper with higher porosity. FIgURE 2c. Variable ink absorption. 18 FLeXO marCh 2011 www.flexography.org FLX_March11.indd 18 3/18/11 1:32 PM