Flat sheet process for production of polyolefin shrink film

Claim 1

A process for manufacturing biaxially oriented polyolefin film with substantially balanced shrinkage of 45 to 65 percent at 120° C. to 140° C. and a shrink force of about 6.9×10 5 to 1.4×10 6 pascal comprising the sequential steps of: (i) conducting a previously extruded and quenched flat polyolefin film of an indefinite length along a predetermined treatment path; (ii) heating a portion of the film in the path substantially evenly to a temperature at least 10° C. above the film's line drawing temperature while not exceeding the film's crystalline melt temperature; (iii) transversely stretching the heated film to a width of from 7 to 10 times the width of the unstretched film; (iv) cooling the film to a temperature below which the film is subject to shrinkage while holding the film in the transverse stretch of step (iii); (v) heating the film substantially evenly to a temperature at least equal to the film's line drawing temperature while not exceeding the film's crystalline melt temperature; (vi) longitudinally stretching the film 1.5 to 4 times the length of the unstretched film so that the ratio of the transverse stretch of step (iii) to the longitudinal stretch is at least 2.0; (vii) cooling the film to a temperature below which the film is subject to shrinkage while holding the film in the longitudinal stretch of step (vi).