Patent ID: 8066924

Claim:
A method of forming a thermoplastic film or sheet comprising the steps of: supplying at least a polymer material P1 and a polymer material P2 to a die, where the die comprises: inlets for the polymer materials P1 and P2, a chamber including: at least one screen ( 301 , 302 ) positioned in the chamber perpendicular to a direction of flow, where each screen includes: flattened apertures sized to reduce an average size of a dispersed phase of the polymer material P1 or P2 in a blend of the polymer materials P1 and P2 and to form flattened fibrils of the dispersed phase in the blend, lamellae ( 303 , 304 , 305 ) interposed between adjacent apertures along their long sides, interconnections ( 306 , 307 , 308 ) portions interposed between adjacent apertures along their short sides, where intervals between interconnections between the lamellae are greater than a distance between each pair of where the apertures form a network of apertures having at least four lamellae interposed between apertures when viewed in cross-section, a gap reduction portion and an exit orifice having an exit gap, where the screens are located at a position in the chamber having a chamber gap wider than the exit gap, and where the gap reduction portion is disposed downstream of the screens and is adapted to dimensionally reduce the chamber gap to a dimension of the exit gap, extruding the blend of the polymer materials P1 and P2 through the chamber, the screens, the gap reduction portion and the exit orifice to form an extruded film having flattened fibrils of the dispersed phase in the blend, where each fibril extends substantially in one direction and where each fibril has a width and a thickness and the width is greater than the thickness, and stretching the extruded film to form a stretched film.