Manufacture of film including integral zipper-like fasteners

Claim 1

In a method of manufacturing film; the steps comprising: extruding a tube of a heat plastified, film forming synthetic resinous material, simultaneously extruding a first thickened profile of a general type defining a continuous gap the width of which is sensitive to hoop tension in the tube during its manfacture, and at least a second thickened profile spaced circumferentially from the gap defining profile and adapted to be interlockingly receivable in said gap by means of cooperative sizing and configuration between the profiles, said profiles extending continuously in the direction of extrusion along the inner surface of the tube, cooling and forming the tube about a trapped gas bubble and thereafter collapsing the tube, the improvement of which comprises, the step of reducing the inherent hoop tension imparted to the gap defining profile by the trapped gas bubble, said step of reducing hoop tension being practiced selectively over a sufficient portion of the region defined between the frost line of the film and that of the profiles as is necessary to properly size said gap respective to the size of the profile receivable therein, to thereby produce from said profiles a mechanically locking and unlocking fastener.