Resin films are used in various fields, for example, for liquid crystal display devices, in view of their chemical characteristics, mechanical characteristics, electrical characteristics, and the like. Specifically, various resin films, for example, transparent protective films for protecting polarizing elements on a polarizing plate are arranged in an image display area of a liquid crystal display device. As such resin films, for example, cellulose ester films which are excellent in transparency are widely used.
A resin film such as a cellulose ester film can be produced using a resin solution (dope) in which a material resin such as a cellulose ester resin is dissolved into a solvent. Specifically, a method for producing a resin film using such dope includes, for example, a solution cast film-forming method. The solution cast film-forming method is used for producing an elongate resin film by casting dope on a running support to form a cast membrane, drying the cast membrane to the degree that the cast membrane can be released, releasing the cast membrane from the support to obtain a film, and conveying the released film by a conveying roller, while performing a drying operation, a stretching operation, and the like on the film.
Furthermore, there has been a demand to increase the size of screens of image forming apparatuses, particularly liquid crystal display devices used as television receivers. Hence, resin films produced for application to image forming apparatuses have been demanded to be wide. A possible method for producing a wide film by the solution cast film-forming method is to use a wide support to increase the width of a cast membrane cast on the support.
However, conventionally, when a wide film is produced by increasing the width of the cast membrane, the resin film obtained tends to be likely to be defective as a result of entrainment of bubbles in ends of the resin film. Even when the resin film is thus defective at the ends thereof, a central portion of the resin film can be utilized by cutting the defective ends. However, there has been a demand to minimize the cut areas in the ends in order to increase the width of the resin film.
Examples of a method for producing a resin film are described in Patent Documents 1 to 3.
Patent Document 1 describes a method for producing an optical film by the solution cast film-forming method, in which a resin solution (dope) cast on a casting surface of a casting support at widthwise opposite ends thereof is lower in viscosity than dope cast on the casting surface of the casting support in a widthwise central portion thereof.
Patent Document 1 discloses an optical film which can be produced to have a large width with the planarity thereof kept uniform so that even the widthwise opposite ends thereof are usable, allowing productivity to be significantly improved.
Furthermore, Patent Document 2 describes a method for producing a propylene film by extruding a propylene resin through a T die having a ceramics sprayed portion formed at an edge of a lip opening in the die by spraying a ceramic material at the edge.
Patent Document 2 discloses that a propylene film can be obtained which avoids being contacted by and mixed with foreign matter adhering to a lip portion surrounding the lip opening in the die.
Additionally, Patent Document 3 describes a method for producing a cellulose resin film in accordance with a molten-resin cast film-forming method, the producing method including discharging a molten resin melted by an extruder from a die onto a running or rotating cooling support in the form of a sheet to cool and solidify the molten resin, wherein a contact portion of the die that comes into contact with the molten resin has a surface roughness of 0.3 μm or less, a contact angle of 50° or more, and a surface energy of 60 mN/m or less.
Patent Document 3 discloses that when the molten resin is discharged from the die, possible streak defects are suppressed which may degrade the surface quality of the film, thus allowing a cellulose resin film with excellent optical characteristics to be produced at a high production efficiency.    Patent Document 1: Japanese Patent Application Laid-Open No. 2009-234169    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-289888    Patent Document 3: Japanese Patent Application Laid-Open No. 2008-194956