A cylindrical screen for perforating a thermoplastic film or sheets has an outer surface and an inner surface. A plurality of outer apertures extend from the outer surface and terminate at a corresponding inner aperture on the inner surface of the screen. The apertures in the outer surface can have...http://www.google.fr/patents/US5718928?utm_source=gb-gplus-shareBrevet US5718928 - Screen for producing a perforated film
