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FLEXO Magazine : July 2008
TECHNOLOGIES & TECHNIQUES 7 6 I _______ __ W 2 8 9 FIGURE 3. SCHEMA OF TRANSMISSION DENSITOMETER. on a viewing table (5). The weakened light is directed by the fiber optics (8) and once again goes through an IR filter (9) or one of colored filters (10) and finally hits a photoconductor II. Today different international standards and recommendations provide different data for quality film reproduction. Usually, the density for offset (solid density) has to be in a range of 3.3 - 3.8D, but flexographic density is typically 4.2 - 4.5D. The density over the whole surface has to be homogeneously distributed. As soon as the film is in use, the core density has to be more than D 4 above the clear film (film base + fog). For instance, we recom- mend the measurement on film in conformance with ANSI/ CGATS.9 dot tolerance for plate making negative film (Table 1). Before and during use, all instruments shall be calibrated in ac- cordance with the manufacturer's recommended procedure. The transmission calibration standard shall be traceable to a standard reference. The clear minimum density area of the film should have a base orthochromatic density less then 0.05D and an ultra- violet density less then O.OID. The black maximum density area of the film should have both densities greater then 4D. The measurement of a minimum density film using an ul- traviolet channel on a transmission densitometer is important to assure optimum proofing and plate exposures. The higher the minimum density the more ultraviolet light will be filtered out. Most proofing systems and photopolymer plates require ultraviolet light to accurately and consistently expose the image. Therefore, the lower the minimum density level the better dot accuracy and dot shape. The verification of dot accuracy can be monitored only if a Table 1. Dot tolerance for platemaking film* known dot percentage is placed on the original file/film prior to duplication or imaging. This is why it is necessary to always use special controlling wedges. In terms of dot shape, the most usable is a round dot. Dot gain is reduced, especially in vignettes and fade-aways. This dot shape has to be accepted for the whole tonal range without exception. Also we recommend avoiding "dia- monds" in the shadows (non printing areas) because they grow and fill-in more quickly. THE COPYING STAGE In case the actual I-percent dots are used, the risk of having problems at the copy stage is quite high, especially when the dot values are in the range of 0.7 to 0.9 percent. Here, small differences (amount of pixels used to build up dots, fog density and even some dust) on the negative film will bring big differences on a plate. We usually consider that under a 0.9 percent on the film (reading value on the densitometer) the reproduction of the dots will start to be a problem. That mea- surement system is valid but suffers from one critical limitation, which is the "zeroing" of the device. In fact, when someone mea- sures the screen percentage, "zero" point always has to be taken by the transparent area of the film. The densitometer then always measures in density but converts density into a percentage. This procedure is correct, provided your film is perfect, and shows a density transmission between 0.04D and 0.06D. Unfortunately, the usual range on production films may vary from 0.05D to 0.08D, not only from film to film but also on the same film. For example, when we take two films with identical design from the same digital file and measure them, Film A shows us den- sity of 0.05D in the transparent and Film B shows 0.08D. The amount of energy passing through the transparent area of Film A will be 2% 10% 25% 30% 50% 70% 75% +/-.5% +/-0.75% +/-1.0% +/-1.0% +/-2.0% +/-2.0% +/-2.0% - J U L Y 2008 *FIRST (Flexographic Image Reproduction Specification and Tolerances) 3rd Edition www.f I exog ra p hy.o rg FLEXO
Flexo Sustainable Fall 2008