Method of manufacture of layered film including layers having crossed directions of molecular orientation

In a method for manufacturing a layered film from thermoplastic material in solid form, such as pellets, powder or amphorous particles, wherein said film is defined by a plurality of superposed layers, the material comprising each layer having molecular chains oriented in a certain direction, the orientation directions of pairs of adjacent layers crossing each other, comprising the steps of heating the solid thermoplastic material to its softening point to obtain a viscous mass of the thermoplastic material;

• conveying the viscous mass while mechanically working the same through at least one flow stream into the annular material shaping die opening of a rotating matrix from which it discharges in the form of a tubular segment whereupon the viscous mass solidifies, and passing the tubular segment as it discharges from the matrix between at least one pair of rotating stretching cylinders to form the layered film;
• simultaneously injecting gas into the space defined within the tubular segment and between the matrix and stretching cylinders whereby the tubular segment obtains a desired width;
• adjusting the rotational speed of the cylinders and the rate of discharge of the viscous mass defining the tubular segment from the opening of the matrix with respect to each other whereby the tubular segment obtains the desired thickness;
• the improvement comprising:
• relocating the frost line band wherein the transition of the material from its molten to a solidified state occurs to a region substantially immediately above the annular die opening through the steps of rotating the matrix at a rate in the range of between about 1 and 20 rpm, and selectively adjusting the stretching speed defined by the stretching cylinders, and the amount of gas injected into the tubular segment, and wherein said relocation of the frost line band is accomplished with the tubular segment not being cooled by separate cooling equipment during the method, whereby the molecular chains are oriented substantially immediately after the thermoplastic material is discharged from the die opening in a molten state and substantially immediately prior to the material solidifying.