Patent ID: 8551372

Claim:
A method for producing a film having a thickness of 20 to 100 μm and a width of 1500 to 4000 mm by a melt casting film producing method, the method comprising: an extruding process of extruding a melted material containing a thermoplastic resin as a main component in a shape of a film from a casting die; a pinching process of pinching the extruded film with pressure between a cooling roller and a touch roller; and an MD stretching process of stretching the pinched film in a lengthwise direction, wherein the MD stretching process comprises heating the film by a heater to soften the film and stretching the film in the lengthwise direction between two conveying rollers which are arranged to have an actual stretching span of 50 to 300 mm therebetween; wherein in the extruding process, a film thickness T 1 (μm) at a central portion and a film thickness T 2 (μm) at both side portions in a widthwise direction of the extruded unstretched film satisfy the following formula: 10 μm≦ T 1 −T 2≦200 μm, and wherein in the MD stretching process, the unstretched film is stretched in the lengthwise direction by 1.1 times to 3.0 times such that after the MD stretching process, the film thickness T 1 (μm) and the film thickness T 2 (μm) satisfy the following formula: T 1 −T 2≦5 μm.