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FLEXO Magazine : July 2008
TECHNOLOGIES & TECHNIQUES FIGURE 4. GENERAL VIEW OF MEASURING DEVICE. much higher than the one passing through the same area of Film B. Prior to using the densitometer we must calibrate it using a special calibration wedge. Nevertheless, those two transparent areas will be considered as the 100 percent of the film, since the densitometer will be "zeroed" on those areas. In the highlight zone, the readings of the minimum dot will likely be similar since those dots have the same diameter (i.e. 1 percent). But in terms of absolute light trans- mission, Film A (i.e. 1. 90D at 1 per- cent) will let much more light pass through than Film B (i.e. 2.09D at 1 percent) and consequently Film A will copy the 1 percent while Film B will not. Another point is absolute density, and the highlights. As explained above, the percentage reading on a densitometer is a very good indication, but relies on the fact that transmission is perfect and equal along the whole film surface. Any fluctuation of the actual film density will have consequences. The only way to ensure a correct reproduction is to measure the amount of energy that passes through the film. This can be done simply by measur- ing the highlight in absolute density rather than in percentage. To establish the "critical density" point, where dot loss occurs, a se- ries of negative films (1 percent - various screen rulings) featuring different densities have been measured and compared. The same "announced 1 percent" density values were ranging from 1.82D to 2.13D. Those films have then been used, for instance, to make a 1.70mm plate under standard conditions, and it has been dis- covered that in all of these cases, all the areas where the density was over 2.00D presented a copy problem (dot losses). A further experiment confirmed that density 2.00D was the criti- cal point under normal platemaking conditions. By overexposing the plate, it has been established that 2.30D is the limit point from which no reproduction will be possible, whatever the exposure time is. As a conclusion of that test, we suggest double checking the film. The first measurement should be a percentage only, and the second measurement should be the absolute density of the highlight zone. It has been determined that the critical area should be around a density of 2.00. Table 2 shows the absolute density values and then an average conversion into percentage. This clearly shows that, in case of slightly lower average density, the risk of having a copy problem for the highlight is tremendously increased. Table 2. Average densities. pending on the screen rulings. With dots less than 20 microns, it is virtually impossible to physically copy accurate dots on a plate. It therefore becomes imperative that we ensure that the negative (used as a master) is free of any and all defects, rather than understand- ing the limitations of these plates. The den- sity in the transparent areas of the film are a first matter of concern; because when density is too high, it will seriously effect the physical light transmission, and thus affect the copy. The resolution (dot per inch) and corre- sponding screen ruling Qine per inch), and the consistency of the finest screen dots are also of concern. A difference of only a few microns will allow some dots to be held, while others won't. As a consequence, miss- ing dots in the screen will occur. In other words, any shift or inconsistency will generate plate problems (for instance, dot loss) because there is not a large enough "safety margin." Even if that assertion is theoretically correct, several counter arguments have to be taken into consideration, at least they do if we speak about conventional processes (using negative films). Generally, it may be considered that 1 percent would, on many occasions, create reproduction problems, for a result that would not necessarily be definitively better. PLATE CONTROL As soon as we get any result on a flexo plate, we should ensure that the image is excellent enough by following this cycle. The different methods to date are as follows: . Visual. An excellent first step for overall surface control, but not thorough enough for getting detailed information and subsequent checks. . Instrumentation. This usually has to be done with an analog microscope and/or special measuring device (Figure 4). This method can be used for intelligent checking of most flexo plates, except plates with aluminum background and sleeves. A combination of all of them is most recommended for all flexo houses. With the help of an analog tool, we can magnify some critical parts and even make some measurements, but in fact using only an analog microscope (even with a special elec- tronic capture option), is not sufficient due to the measuring time and handling. This is why special equipment is so necessary. Another argument could be special software which helps to detect dot area, dot sizes, screen ruling in automatic mode, etc.. We can also apply manual measurements for some critical situations where math engines cannot properly de- tect any dots. It is a typical situation with small dots and in the highlight area. Figure 5 shows what kind of picture device "grabs" from the surface. Depending on the thickness of a plate, the recognition becomes critical to ensure the clarity of the picture. That the picture is unclear indicates difficulties that have emerged and underline the necessity in perfect plate production. Density values 1.90 - 2.00 1.50 1.27 1.00 0.34 0.09 0.04 - 0.06 Average % conversion 1% 3% 5% 10% 50% 90% 100% ACHIEVING AND HOLDING 1% It has been established that flexographic plates can reproduce extremely fine screen dot sizes (between 20 and 30 microns). These dot diameters correspond with approximate I-percent dots, de- - J U L Y 2008 www.f I exog ra p hy.o rg FLEXO
Flexo Sustainable Fall 2008