Abstract: A process for continuously heat setting a tubular film of biaxially oriented polyethylene terephthalate in a state expanded with a gaseous material and taking the heat set film up on a winder which comprises carrying out the heat setting in a heat setting chamber wherein the temperature is controlled so as to be initially in the range of about 220.degree. C.-240.degree. C. in the first zone of heat setting and thereafter declines to about 195.degree. C.-215.degree. C. in the final zone while the length of the film is increased about 2% - 7% and the diameter of the film is reduced about 5 - 15% from the corresponding dimensions of the film before heat setting, the film then being taken up by a rotary winder.

Abstract: Method for preparing a biaxially oriented polyvinyl alcohol film by extruding a mixture of polyvinyl alcohol and water, and if desired, incorporated with a polyhydric alcohol, in a tubular form, drying the tubular film to give a tubular non-stretched film having a degree of crystallization of not more than 44 % and a water content of not more than 20 % by weight, and biaxially stretching the non-stretched film at a stretching ratio of not less than 2 in each of the machine and transverse directions by means of inflation under conditions such that the film surface at the starting point of stretching is maintained at a temperature in the range from about 60.degree. to about 150.degree.C., the surface of the film reaching a maximum temperature at the middle portion of a stretching zone extending from said starting point to a point where the tube reaches a maximum diameter and the difference between maximum and minimum surface temperatures in the stretching zone does not vary more than about 15.degree.C.