Inspection printed films

Inspecting printed films in the wet state is somewhat difficult because of the high reflectivity of the film. In the dry or fired state, inspection is easier as there is more of a contrast with the substrate. For laboratory or small manufacturing operations, a microscope with backlighting is an essential tool. It is very easy to see voids or thin areas because alumina and beryllia are translucent to a certain degree. However, for multilayer applications or thicker substrates, this method is inadequate, and one must revert to inspection using top lighting. 

Photomechanical stencils are of two types direct coatings and transfer films. Direct coatings are either bichromated gelatin or bichromated poly(vinyl alcohol) (PVA). The coated screens are exposed through a positive, washed, and inspected. These screens are used for printing electronic components. They are not practical for commercial work because of the difficulty of reclaiming the screen after use. 

A large fraction of the substances used in inks will not permeate through the printed substrate because their molecular size is large and so diffusion is slow. These substances will not come into contact with (and possibly contaminate) the packaged food unless set-off occurs when the printed surface touches the food-contact surface, such as when films are stored on reels or when printed containers are stored stacked or nested. Of course, printers constantly inspect the quality of their work and are alert to the possibility of set-off. But visual examination will detect only the coloured pigments and dyes. The uncoloured, invisible, components of inks go undetected and there is a need to minimise any set-off of these invisible components onto the food contact surface of the packaging material. 

---