Method for manufacturing a flattened cylindrical film consisting of layers of thermoplastic resin stretched lengthwise and crosswise

A method for producing laminated tubing comprising the steps of simultaneously extruding an outer, an intermediate and an inner tube of plastic material concentrically with at least said outer and said inner tube radially spaced from each other, said intermediate tube being extruded from a material having a lower melting point than the other two tubes;

• drawing said outer tube over a first mandrel so as to stretch said outer tube only in longitudinal direction;
• drawing said inner tube over a second mandrel which diverges in said longitudinal direction so as to mechanically stretch said inner tube in a direction transverse to said longitudinal direction to a diameter substantially equal to that of said outer tube;
• drawing said intermediate tube over a third mandrel so as to stretch said intermediate tube in both longitudinal and transverse directions;
• and heating said mandrels and transmitting heat from the same to said tubes during the stretching thereof so as to weld said outer tube stretched in said longitudinal direction, said intermediate tube stretched in said longitudinal and said transverse direction, and said inner tube stretched in said transverse direction together by heating said intermediate tube close to its melting point while pressing said tubes against each other.