POLYESTER FILM AND PRODUCTION THEREOF

PURPOSE: To obtain a polyester film excellent in low heat shrinkability and prevented from the deterioration of flatness at the time of pressure feed accompanied by thermal softening by setting the heat shrinkage factors in the longitudinal and lateral directions of the film to a specific ratio and setting the penetration distance of a pin at a time when the pin is pressed to the film under a specific condition to specific % or less of the thickness of the film. CONSTITUTION: The heat shrinkage factors in the longitudinal and lateral directions of a film are 0.3% or less and, when a pin with a leading end diameter of 0.8mm is pressed to the film under such a condition that pressure load is 10g and a temp. rising speed is 5 deg.C/min, the penetration start temp. of the pin to the film is 115 deg.C or higher and the penetration distance of the pin is set to 8% or less of the thickness of the film. This film is subjected to high temp. heat treatment for imparting low heat shrinkabillty, flatness and thermal softening resistance but, in the case of polyethylene terephthalate, rapid temp. rising short-time treatment is performed at 225-255 deg.C. After the film is cooled to about 210 deg.C or lower under tension, the film is pref. subjected to relax treatment because relaxation of molecular orientation can be suppressed and thermal softening resistance is imparted.