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FLEXO Magazine : November 2009
36 FLEXO NOVEMBER 2009 www.flexography.org than six months in the direct sun, have been developed with excellent rheology across the color gambit, the only excep- tion being clean reds and violets. Even the intermediate-fade resistant (IFR) pigments (less than a year in the sun) have excellent flow. But when it comes to the extended-fade resistant (EFR) pigments, the flow starts to suffer. This is true for both water and UV. In general, more resistant pigments tend to be either inorganic compounds, or are larger molecules, leading to more difficulty in developing stable dispersions. The nature of the pigments, along with the difficulty in processing, and the fact that many of these lightfast pigments are weaker in strength, leads to a net result of significantly increasing the cost of the inks. A typical NFR inks to IFR pigments would only entail a fractional increase in cost, mostly involving changes in the yellows, oranges, and reds. A further upgrade to EFR pigments could result in a doubling or even tripling of the costs. That being said, with UV inks there is really just two steps between NFR and EFR. Due to the cost of the other ingredients (photoinitiators, monomers, etc.) there is really no need to have an intermediate fade resistant version of inks. The biggest factor that prohibits the printer from using high-line aniloxes everywhere is the stock. With films, and extremely smooth-coated high-gloss and semi-gloss papers, higher-line aniloxes are easily used. However, on porous and rough substrates, the higher-line aniloxes simply do not deliver enough ink volume to competently cover and produce a smooth uniform appealing consistently measurable color (see Figure 3b). Under these circumstances, it is necessary to drop down in anilox for the solids so that there is enough volume of ink to cover the material (see Figure 1b). Although uncoated paper is not used as frequently today, the same results can be seen, particularly with thermal transfer stock (see Figure 2b). With UV, the issue is absorption. When the press runs too slowly, on paper substrates the ink will dive into the stock. This diminishes the ability of the ink to cure and creates a very unappealing mottled visual appearance. For highly absorbent stocks, the inks have to be formulated to provide better hold- out, usually by means of increasing the viscosity or by using a UV transparent filler of some sort. PUSHING THE LIMITS All this leads us to a question on the true limits of flexo tech- nology. If we all printed on stable films or on highly coated stocks, would there still be a limit? Well, there are limitations to what can be done from an ink formulation standpoint. First, it is important to discuss the neighborhood in which we live. The anilox, often described as the heart of flexo, is used to transfer ink to the plate. It used to be that a particular line-screen would have a relatively consistent volume from one manufacturer to another. Lately, however, advancements in anilox technology have resulted in quite a different story (see page 62). So, we hope that everyone is on board with the knowledge that the line-screen is no longer sufficient in terms of discussing anilox requirements. What is important is the volume, and even more important is the volume of the ink transferred from the anilox to the plate. So, if we use two aniloxes as examples, say a 1,500lpi (1bcm) versus a 500lpi (4bcm), the 4bcm anilox has four times the amount of ink in the cells, and even if the inks transfer equally out of both aniloxes, the finer-line anilox is expected to transfer to the substrate an ink that provides the same level of performance as the 4bcm anilox. In order to achieve this, the formulator must adjust the formula. The most obvious result is a necessary increase in pigmentation. The degree of technical skill in dispersions can vary among producers, but typical dispersions used for flexo printing average around 35 to 40 percent pigment loading with pigment particle sizes in the 0.25 micron range, especially for process work. If we have a particular color match that requires, say 30 percent dispersion, to meet the standard with a 4bcm anilox, a certain amount of volume in that anilox is committed to pigment. If we now reduce the volume of the ink by a factor of four, it makes sense that the amount of dispersion must increase. In the example above, one can hardly formulate an ink with 120 percent of the dispersion. So, work must be done to make stronger, i.e. higher pigment loading, dispersions. However, the amount of pigment, keeping the particle size constant, that makes its way to the substrate needs to remain consis- tent. So, if we increase the pigment loading by 50 percent, we have a shot at matching the color. This is easier said than done. The current state of the industry for dispersions that are still flowable is limited to about the 45 to 50 percent pigment range, depending upon the pigment. Even if it can be done, it will require a much higher loading than the 30 percent level needed for the 4bcm anilox. What will it replace? How about the solvent? If the viscosity must remain fixed in order to accommodate pumping and/or drying issues, that solution might not be possible. How about the resin? If adhesion is critical, pigments do not add much in terms of adhesion. You need the resins for this, so reductions in the level of resins could lead to adhesion issues. So, what your father always told you about using the right tool for the right job still applies for anilox selection. You may want to use the finer line aniloxes in order to take advantage of the lower dot gains, improved highlight reproduction, and better ink mileage, but you had better have tools in house to keep these aniloxes clean so that your process is repeatable. This will require cooperation with your ink supplier to provide you with the inks of the appropriate strengths, along with some practical understanding of the limitations of the particu- lar ink system involved. FIGURE 3A. High pig- mented ink printed us- ing 800lpi/3bcm anilox on semi-gloss. FIGURE3B. High pig- mented ink printed us- ing 800lpi/3bcm anilox on uncoated. FIGURE 3C. High pig- mented ink printed us- ing 800lpi/3bcm anilox on high gloss. TECHNOLOGIES & TECHNIQUES
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