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FLEXO Magazine : July 2007
hot technologies w w w . f l e x o g r a p h y . o r g J U LY 2 0 0 7 F L E X o 2 1 theoretical volume of ink that the anilox can carry is actually transferred to the plate after doctoring, and then later to the sub- strate. This transfer factor is dependent both on ink formulation and on the anilox/cell configuration. Increasing the UV flexo ink layer to simulate screen white requires new engravings which give a high cell volume. This, together with specially developed viscosity and flow properties in the ink , enables the transferable volume of ink to achieve in- creased density/opacity. Recent research shows that ink formulations and anilox en- gravings have greatly improved the ink transfer factor—thus enabling higher ink film weights and the ability to provide solid areas and text at commercially printability level. RotaRy ScReen Laydown The ink deposit, and thus ink film weight in the rotary screen process, depends on the screen mesh configuration. There are many different manufacturers of screen mesh types. Research done for this article used Stork screen. Similar results have been achieved with Gallus material. To determine deposit, the screen measures the number of lines per inch in the mesh, the thickness of the ink layer, and the per- centage open area. Again, the final deposit of ink on the substrate and the structure of the laydown (i.e. no pinholes) are dependent on both screen mesh configuration and ink formulation. In the past, the major draw back to using screen white in combination with UV flexo ink was the need to modify the UV flexo ink with silicone to wet out over the screen white. With the introduction of silicone-free combina- tion whites, this issue has been eliminated. An example of a red UV flexo ink over silicone and silicone free combination white is shown in Figure 5. The main drawback for label convectors using screen white has always been press speeds. In most cases, flexo printers/converters who run combination flexo/screen labels for “no-label look” designs are limited to 200 – 300 fpm due to the fact that screen inks need a certain amount of dwell time to completely flow out and cure. PRintabiLity Can UV flexo replace UV screen? After review- ing the fundamentals of both processes in relation to laying down an opaque white ink, it is evident that there are limiting factors in the transfer pro- cess of flexo which makes it difficult to apply the right amount of ink to the substrate to completely replace screen white. However, recent studies show that in solid areas it will be possible for UV flexo to match the opacity of rotary screen; limitations will be evi- dent in finer negative text, where rotary screen still has an edge. Screen printing can create sharper images at lower point sizes. Extensive testing using different anilox roll and mounting tape combinations demonstrated that, for type sizes as low as eight point, UV flexo is commercially acceptable for reversed-out text. This has been proven by converters using this technology com- mercially today. MeaSuRing the diffeRence The opacity of an ink can be measured using a method called contrast ratio. This is a relative number, whereby a black density is measured through the layer of white ink. Total transparency will give the number one, and total opacity will yield 100. To quantify if a UV flexo white can “measure up” to rotary screen white, the following series of tests were run comparing screen white using a 305 13-percent opening screen to standard UV flexo white and a new UV flexo white optimized for opacity and flow. Both flexo whites were run on three different banded anilox rolls with different cell volumes and cell configurations using several plates and mounting tapes on a pressure sensitive PS BOPP material. During this research, it was discovered that a complex set of factors impact the transfer process. Table 1 shows the components in the process that were optimized to achieve the best test results. ink foRMuLation aniLox tyPe doctoR bLade Pigment type Cell Configuration tested: Pressure Content Screen / BCM / Angle Angle Flow / viscosity 250 12 45 Thickness 60 25 30 Channelled Stiffness 66 22 30 Channelled Material 85 20 30 Channelled 120 15 30 Channelled 100 18 30 Channelled 120 15 Open Cell 85 20 Open Cell 60 25 Open Cell taPe PLate PRinting PReSS Compressibility Type Press speed Thickness Shore hardness Impression, plates to anilox & plate to substrate. Thickness PARAMEtER CoMPARISoN figuRe 3.+#3; Anilox engraving method, pattern, angle, channelled, not channelled and open celled all play a part in how much ink
Golden Anniversary Commemorative Journal