PREPARATION OF BIAXIALLY ORIENTED POLYESTER FILM FOR MAGNETIC RECORDING MEDIUM

摘要

PURPOSE:To obtain a biaxially oriented polyester film for a magnetic recording medium excellent in dimensional stability and strength by a method wherein the tab end parts of a film are cut immediately before the film is separated from the clips of tenter chains to thermally relax the film, and the surface temp. of the film and the tension of the cut parts on the side of a take-up roller are respectively specified. CONSTITUTION:Polyethylene terephthalate is extruded in a molten state to be molded into a sheet which is, in turn, stretched in the longitudinal direction to form a longitudinally stretched film Fa. This film Fa is supplied to a tenter to be preheated in a preheating zone A and subsequently stretched in a lateral stretching zone B to obtain a biaxially oriented polyester film Fb. Heat fixing treatment and relaxing treatment in a lateral direction are applied to said film Fb in a heat fixing zone C and the film is continuously passed through a constant temp. setting zone D to accumulate heat. Thereafter, the tab end parts of the biaxially oriented film Fb are cut under such a condition that the surface temp. thereof is glass transition temp. (Tg)-Tg+80 deg.C and the cut film is taken up under tension of 0.48-1.90kg/mm<2> by a take-up roller 5 to obtain a biaxially oriented polyester film Fc.