Process for preparing biaxially drawn thermoplastic polyester film

In a process for preparing a biaxially drawn thermoplastic polyester film by concurrently-biaxially drawing a thermoplastic polyester film by a tenter technique, the improvement comprising the sequential steps of:

• (a) preheating a thermoplastic polyester film in a preheating zone to a temperature higher than the temperature at which the film is to be drawn;
• (b) concurrently biaxially drawing said preheated film at a temperature greater than the glass transition temperature of the polyester;
• (c) heat treating said drawn polyester film in a heat treating zone at a temperature less than the preheating temperature;
• and
• (d) progressively heating said heat treated film at increasingly higher temperature levels in a plurality of steps in a reheating zone, wherein the increments of temperature increase between progressively higher temperature levels are about the same.