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FLEXO Magazine : January 2008
. . use 0.045-in. to save on the cost of the polymer. Understand, though, that switching thickness usually requires new print cylinders or using tape to build up thickness. Plate hardness is another important factor when selecting plate material. The unit of measure for hardness is Shore A, measured with a durometer. Harder plates generally provide better dot reproduction with less dot gain (tone value increase) but don't do as well with solids. Similarly, hard plates do well on very smooth substrates but struggle with rough surfaces. Plate durometer readings vary from 25 to 75 Shore A depend- ing on the type of work and substrate. Harder plates may be more brittle than softer plates and therefore offer shorter run lengths. IMAGING Today, there are more imaging processes for flexo plates than ever before. Polymer plates are imaged in one of five ways: a) a film mask [ analog]; b) a carbon mask; c) a transfer mask; d) an ink-jet mask; e) direct ablation of the material. Rubber plates are either molded or directly ablated. - - - - - - ::::::: - - - --.... - " .-; :-=' =. -::.- , "=Þ -- "20 A :.-;. ';. ":.. ':. .. . .. . ... t ..':" t.-.....-- ;..-'";.....-;- ----- ..,..( \ -\ "',- -.... ,..0 ';1,. . - - -....-... ... þ ''ã Q ': ':. .. ... - .. ' . . . If using one of the mask systems, you must expose the plate through the mask to form the graphics. This is what creates the hard part of the plate-the part that remains after processing. When handling a carbon mask plate, you must use care to ensure you don't scratch or kink the pre-coated plate mask. This means storing the plate on good shelves (boxes flat and not stacked too high) and supporting the plate when moving it. Do not remove the protective coating until just prior to imag- ing the carbon coating on the laser imager. The typical sequence for making carbon mask plates is: 1) Back exposure to create the floor thickness 2) Carbon mask imaging to ablate the carbon and form the mask 3) Main exposure to expose through the mask and polymer- ize the image area 4) Plate processing to remove the unexposed, unpolymer- ized material 5) Quality check of relief, dot area, durometer, etc. 6) Post exposures and detack to finish the plate. Various systems have been used for exposure. Bank exposure systems (where 20 or more UV lamps are simultaneously il- luminated) have been around a long time and remain the most common today. In the 1990s, point light sources gained popu- larity for their ability to produce a dot with sharper shoulders, and subsequently finer printed dots with less dot gain. These systems have a single lamp and produce more collimated light (hence the sharper shoulders). This provided a distinct advan- tage in analog, film workflows. However, one of their draw- backs is the potential for dot pointing, where each dot on the plate points to the light, much like a plant points to the sun. This isn't a problem when the lamp is directly above the dot, but for dots near the edge of the plate, this can be problematic as the dots angle toward the lamp. With film, the benefits out- weighed the potential problems. With carbon mask systems, bank sources are generally preferred over point light sources. Because of the intimate contact of the carbon (compared to film) and the elimination of the diffusion sheet (used to create vacuum with film), bank systems provide shoulders that are much sharper than those same exposure systems using film. Even though the operator doesn't have as much latitude compared to point light systems (which allow the operator to decide how much direct light and how much diffused light is used), the results are close to the benefits found in point light sources. Further, the bank lamps reduce the dot pointing problems with point lights, which is exaggerated by the intimate carbon mask. Warming up bank lamps is crucial for repeatable results. Point lamps are on whenever the unit is powered and use a shutter system that opens during the exposure. However, bank lamps illuminate only when the exposure timer is started. This results in radically different UV output when cold lamps are initially started. Make sure your lamps are warmed for at least two minutes before an exposure if they have been cold for a while. Also, with 20 or more lamps, they will not age at the www.flexography.org JANUARY 2008 - FLEXO