There are three processes for film production depending on the polymer used. These are extrusion from the melt, stretching (mono- and biaxial) of a deformable solid, or casting of a polymer solution.  Casting is used with polymers that do not melt or which only melt under disintegration, like polyimides.  Casting processes are also of interest for the production of ultra-thin films from thermoplastic polymers, like polyester or polycarbonates. Filtration is an essential step within film production to pre-treat melts or polymer solutions. Depth filters retain undissolved polymers or gels from solutions, which would be detectable as specks within the cast film. These specks would significantly reduce the tear resistance of the films.  The production of ultra-thin films for challenging applications, e.g. in medical technology, is made possible by the high level of purity of the filtered melts and solutions.