1. Field of the Invention
The present invention relates to an apparatus and a method of producing a multi-layer film.
2. Description Related to the Prior Art
Recently, a demand of a transparent thin film increases since the thin film is used as an optical film, such as a protective film for a polarizing filter of a liquid crystal display, an optical compensation film in an optical retardation filter and the like. These optical films are required to have a low optical anisotropy, a smoothness, a high resistance and the like, and preferably produced from cellulose triacetate.
In order to produce the optical films, a film producing apparatus for producing a multi-layer film is preferably used, to which a method of casting a multi-layer solution is applied. In the film producing apparatus, a higher viscosity polymer solution and a lower viscosity polymer solution are fed through respective feed paths and joined in a joining section of a feed block to form the multi-layer flow in which the higher and lower viscosity polymer solutions flow in parallel. The multi-layer solution is discharged from a casting die and cast onto a support so as to form a casting film having a multi-layer structure. The casting film is peeled from the support. Then edge portions of the casting film in a widthwise direction is slit off or trimmed off, and a middle portion of the casting film is obtained as a production film having a multi-layer structure.
Recently, in the joining section of the feed block, the higher viscosity polymer solution is often sandwiched between the lower viscosity polymer solutions. In this case, the casting film has a multi-layer structure constructed of an inner layer (or intermittent layer) formed from the higher viscosity polymer solution and outer layers (or a surface layer and a back layer) formed from the lower viscosity polymer solution.
In the prior art, as shown in FIG. 12A, a first solution 100 and second solutions 101 are joined to form the multi-layer flow in which the first and second solutions 100, 101 have the same width. However, as shown in FIG. 12B, the first and second solutions 100, 101 in multi-layer flow is simultaneously cast to form a casting film 104 of a multi-layer structure with an encapsulation in which outer layers 102 of low viscosity cover an inner layer 103 of high viscosity. In this case, edge portions of the casting film 104 contain a large amount of the solvent, and therefore often cannot be dried enough. Thus when the peeling is made, some parts of the edge portions remains on the support, and the casting film 104 is torn from the edge portions, which causes the stop of film production. Further, when the drying is made, voids are generated in the edge portions. Thus the casting film is torn from the part in which the voids are generated, and the film production stops.
In an apparatus for producing a multi-layer film disclosed in the Japanese Patent Lai-Open Publication No. 2002-221620, a joining section of a feed block is provided with a distribution pin on which a groove is formed, and a plan of the groove has a trapezoidal shape. In the groove the outer solutions is fed, and thus as shown in FIG. 13A, the width of the outer solutions 105 becomes smaller than the inner solutions 106 after the joining. Note that the widths of the groove can be changed by rotating the distribution pin.
However, even if the distribution pin in which the groove is formed as described in the above publication is used, the covering phenomena occurs, and there are acceptable covering phenomena and unacceptable covering phenomena. Especially when the production speed is made higher so as to increase the productivity, the unacceptable covering phenomena occurs, and the remaining part of the casting film on the support after the peeling becomes larger. Therefore, it is necessary to determine a size of the groove and a width of the solution for forming the outer layer strictly.
Further, when the trapezoidal grooves are formed as described in the above Publication, as shown in FIG. 13B a middle part of the discharged solution 107 for forming the outer layer becomes much thinner, and therefore the thickness becomes nonuniform in the widthwise direction. Furthermore, since a depth of the groove depends on the rotational position of the distribution pin, a difference of the flow speed between the inner solution and the outer solutions in the joining section increases depending on the depth of the groove. Thus the inner and outer solutions are unstably fed at the joining, and therefore the thickness of each layer becomes nonuniform in the feeding direction.