Process for preparing biaxially drawn polyamide films

Abstract

Biaxially drawn polyamide films having an excellent levelness are prepared as follows. A substantially unoriented polyamide film is heated at 45.degree.-65.degree. C., and then drawn in the longitudinal direction at a deformation rate of at least 10,000%/min. and a draw ratio of 2.7-3.5 by a roll stretcher. The drawn film is immediately transferred to the lateral drawing position at 45.degree.-60.degree. C. within a time (t):t=e.sup.(3.9-0.053T.sbsp.1.sup.) where e=the base of natural logarithm and T.sub.1 =film temperature during this period of time (t), which is 45.degree.-60.degree. C. The thickness profile of the film at the starting position of lateral drawing is gradually reduced from the side and portions toward the central portion of the width so that the film thickness at the central portion is 75-90% of the film thickness at the side end portions. The film is drawn in the lateral direction at an average deformation rate of 2,000-10,000%/min. and a draw ratio of 3-5 by a tenter type stretcher in such a manner that, until the mechanically set draw ratio between respective tenter clips exceeds 1.4 times the original distance, the film is expanded at an angle of up to 6.degree. with respect to the travelling direction at a tenter clip temperature T.sub.2 (T.sub.2 <T.sub.1). The film temperature is elevated stepwise in the lateral drawing and the film temperature at the termination position of lateral drawing is 70.degree.-100.degree. C.