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FLEXO Magazine : February 2010
FLEXO FEBRUARY 2010 www.flexography.org plate thickness. Uneven plate thickness results in the press operator increasing impression to the lowest point on the plate to achieve ink transfer. This over-impression then results in pre-mature plate wear. Finally, the platemaker determines the minimum time re- quired to post expose (UV-A) and detack, or light-finish (UV-C). Post exposure ensures that all material on the plate is com- pletely exposed, while the exposure to UV-C is to make sure that the tackiness is removed from the plate. Both processes affect durometer (hardness) of the plate along with the surface tension of the face. Hardness and surface tension both affect the ink releasing characteristics of the plate. When the plate is out of spec, then the press operator is left to try to compensate through non-standard ink and impression settings. PLATE MOUNTING If prolonged exposure UV light is a possibility in the mount- ing area, UV blocks should be utilized. Often it is the mounter's responsibility to trim the plate to fit the cylinder. When a large plate contains multiple staggered lanes, the result is often a zig-zag or jagged cut. At any point where two cuts meet at a right angle, a punch should be used to provide a radius at the corner of the two cuts. This radius will allow for easier plate removal and minimize the likelihood of the plate tearing. Sticky- back, or mounting tape selection also has an impact on plate life when combined with the uniformity of the plate and the expertise of the press operator to achieve impression settings. The compressibility of the mounting tape is typically dictated by the type of print required, i.e. solids, combo, or screens. The more compressible the tape, typically the less force the plate takes. However, if the compressibility results in pin-holing, then the operator's typical response will be to adjust impression, thus affecting plate life. Once the plate is mounted and staged for pressruns or cleaning, it should be wrapped in a black or opaque poly with the edges sealed to protect against UV light and ozone. In years past, mount- ers and press operators alike have used release sprays in screened areas of photopolymer plates in hopes of minimiz- ing dirty print. While some people do find this beneficial, it is important to remember that the sprays used are temporarily changing the surface energy of the plate face and therefore could adversely affect ink release. PRESS CREW One of the most vulnerable times for plates to be damaged is when putting print cylinders in and taking them out of the press. Often in wide web CI (central impression) applications, the drain from the ink pans comes in close contact with the mounted printed cylinder, thus extreme caution should be taken. While it is not always feasible to block all UV light in a pressroom, one area that can be addressed is between-color lighting that comes in very close contact with the photopoly- mer plates. Setting final ink and impression settings at press speed will help in achieving the kiss impression needed to maximize plate life. Understanding your ink system, extenders and cleaners is important in maintaining not only plate life, but also color, viscosity, and pH. Some solvents are not compatible with pho- topolymer plates above certain levels. Adverse effects to the plate from some solvents include swelling, shrinking, cracking or softening. Pure, 100-percent esters, ketones, and glycol ester should be avoided. However, some can be considered used as a co-solvent with normal propyl alcohol at a much di- luted dose. While some consider the use of aliphatic/aromatic hydrocarbons acceptable if used as a co-solvent at 1 percent to 5 percent, it is a good idea to avoid these altogether. Most alcohol/glycols are acceptable for use with photopolymer plates, with a few exceptions which include undiluted octyl, benzyl, and methyl. When adjusting pH, most amines are compatible, with the exception of morpholine. A swell test can be performed to determine the level of compatibility of the solvent and the photopolymer plate. If swelling is less than 50 microns and there is a loss of less than 3 Shore A, then the solvent would be considered resis- tant. If swelling is 50 to 80 microns and there is a loss of 3 to 7 Shore A, then the solvent would be considered conditionally resistant. If swelling is greater than 80 microns and there is a loss of greater than 7 Shore A, then the solvent would be considered non-resistant. It is in the pressroom that ozone is often most apparent. Ozone (O3) is a highly reactive gas compound composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level. In areas such as a pressroom, the most likely generators of ozone are electrical motors and corona treaters. If you experience plate cracking attributed to ozone, you can contact your plate maker or photopolymer supplier and request an assessment of the area in question. Ozone measur- ing devices show ozone levels in parts per billion. CLEANING AND REMOVAL In order to maximize plate life, plates should be cleaned immediately after the pressrun. Plates should be cleaned with a lint-free cloth that contains no harmful debris, or a soft natural bristle brush to safely remove ink in screen and pro- cess areas without damaging the highlight dots. Do not use nylons to clean plates. The nylon material acts as a grater in fine highlight dot areas. NP alcohol may be used sparingly to clean polymer plates. However, plates should not be soaked in any cleaning solution. Acetate should not be used to clean plates. However, if acetate is used press-side or for cleaning purposes, it should be used in a reduced solution no greater than 20 percent. Plates are to be completely dry before stack- ing and stored flat. Plates should not be rubbed in order to dry. Blot or blow them dry, using a lint-free towel or cloth. Extreme care is required when demounting photopolymer plates. Some plate manufacturers supply demounting tools which assist in the removal of the plate from the cylinder. No sharp objects or knives should be used, as they could damage the plate. Starting at the edge of the mounted plate, slowly pull the plate from the mounting tape while working your way across the print cylinder. Once the plate has an ex- posed flap across the print cylinder, grasp and peel the plate Excess plate relief results in image areas lacking support and premature wear, especially in screen and process areas.