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FLEXO Magazine : July 2008
TECHNOLOGIES & TECHNIQUES FIGURE 2. SCREEN DOT ON A SOFT-DOT FILM, SHOWING FUZZY EDGES (LEFT) AND SCREEN DOT ON A HARD-DOT FILM WITH SHARP EDGES (RIGHT). process. The structure of graphic films is quite complex and con- sists of many layers (Figure 1). The main components of graphic films are: . Polyester film (PET), which is the base layer of the whole film. Traditionally this film is 100 or 17 5 of thickness. . Sub-layers are coated layers present on both sides of the PET that ensure the adhesion of the emulsion and the back layers. . Emulsion is the light-sensitive part. It contains the silver halide crystals. This layer is where the image will finally form (together with the protection layer: :t 4 m thickness). . Protection layer is on top of the emulsion (single in case of clear film/double with extra matting agent in the case of matt films). . Back layers or antihalation layer (:t 3 m thickness). Both types have the same structure, the difference being only that the matt films are actually "clear" and have been coated with a second layer of large matting particles. The difference between matt and clear films is found only in the second protection layer (the emulsion itself being the same). This means that the sensitiv- ity of both films is theoretically the same. In practice, the matt film will feature a slightly different result, as there can be some light diffraction in the matt protection layer during the film plotting. The difference, as it turns out, becomes quite negligible. The protection layer of the "clear film" contains very small Anti- Newton effect particles in size of -2 . The "matt film" features a second protection layer that contains larger matting particles -7 . This matt aspect is a rule of thumb for photopolymer platemak- ing. It helps the continuous evacuation of a gas trapped between the film and the plate during the face exposure. This is the only way to ensure good vacuuming and an excellent contact between negative and photopolymer plates. As such, it ensures a correct reproduction of the image onto the plate. - The physical appearance of a matt film is largely determined both by the amount of matting agents used, and by the size of particles used. Another issue to consider is the "sharpness" of the films. Two types of films can be used: the soft-dot films (also called "rapid access") and the hard-dot films. The rapid access is considered the standard type of negative film used in flexography. These films are widely used and are showing major benefits, such as larger developing latitude. In comparison with hard-dot films, they are also less sensitive to temperature, exposing time, etc. On the contrary, hard-dot films, which are more sensitive to these factors, feature narrower lati- tude of exposure and take longer to develop. Additionally, they have a higher practical density and a better image sharpness than the soft-dot films (Figure 2). The sharpness of these dots also has some impact on the dot size of exposed photopolymer plates. It has been observed that hard-dot films feature a little less dot gain on negative plates, than the rapid access films exhibit. CONTROLLING THE FILM Most companies are using transmission densitometers to properly control films. The primary use of these densitometers is RIP calibration, but they are also used for checking films after exposing and washing. Often operators mistakenly use these instruments for calibration only. We have always considered that the development process drastically influences the final film quality. For this reason we deem it necessary to always check both final films and RIP cali- bration wedges. Technically, the principle of any transmission densitometer is quite simple (see Figure 3). The light from the source (2) is reflect- ed by a special mirror (1) and then reflected at a 90-degree angle by another mirror (3). Next, the light passes through the heat filter (4), fixed aperture (6) and checked film (7), which is placed J U L Y 2008 www.f I exog ra p hy.o rg FLEXO
Flexo Sustainable Fall 2008