Patent Publication Number: US-2009218722-A1

Title: Casting device, and solution casting method and apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to a casting device, and a solution casting method and apparatus. 
     BACKGROUND OF THE INVENTION 
     A polymer film (hereinafter referred to as film) has advantages such as excellent light transmission properties and flexibility, and is easy to be made lighter and thinner. Accordingly, the film is widely used as an optical functional film. As a representative of the film, a cellulose ester film using cellulose acylate or the like has excellent toughness, and phase difference is small in the cellulose ester film. Therefore, the cellulose ester film is utilized as a base of photosensitive material. Additionally, the cellulose ester film is utilized as a protective film in a polarizing filter or an optical compensation film as a component of a liquid crystal display (LCD) whose market is increasingly expanded recently. 
     As a film production method, mainly, there are a melt-extrusion method and a solution casting method. In the melt-extrusion method, a polymer is heated to be melted, and then extruded by an extruder, to form a film. The melt-extrusion method has advantages such as high productivity and relatively low equipment cost. However, in the melt-extrusion method, it is difficult to adjust thickness of the film with high precision, and further fine streaks (die lines) easily occur on a surface of the film. Accordingly, it is difficult to produce a film having high quality as an optical functional film. On the contrary, in the solution casting method, a polymer solution (hereinafter referred to as a dope) containing a polymer and a solvent is cast onto a support to form a casting film. The casting film is hardened enough to be peeled and have a self-supporting property, and then peeled from the support to form a wet film. The wet film is dried to be a film. In the solution casting method, it is possible to obtain a film having more excellent optical isotropy and thickness evenness and containing less foreign substances in comparison with the melt-extrusion method. Therefore, the solution casting method is mainly adopted for a producing method of an optical functional film for use in the LCD. 
     In the solution casting method, the dope is prepared by dissolving a polymer such as cellulose triacetate into a mixed solvent containing dichloromethane or methyl acetate as a main solvent. Then, a defined additive is mixed with the dope to prepare a casting dope. The casting dope is cast through a casting die onto a support such as a casting drum and an endless belt to form a casting film (hereinafter referred to as a casting process) The casting film is hardened enough to be peeled and have a self-supporting property on the support. Thereafter, the casting film is peeled as a wet film from the support. The wet film is dried and wound as a film. 
     Recently, in accordance with rapid increase in demand for the LCD and the like, a solution casting method having high production efficiency has been desired. In view of increasing the production efficiency, the speed at which the casting process is performed is slowest in the solution casting method. Therefore, for the purpose of speeding up the solution casting method, the moving speed of the support is made faster, and an upstream side from a casting bead in the moving direction of the support is decompressed by using a decompression means such as a decompression chamber. Note that, the casting bead is the casting dope extending from the casting die to the support. 
     During the casting process, when the clearance between the support and the decompression chamber is changed, the following problems occur in some cases. In accordance with pressure fluctuation inside the decompression chamber, a position of the support where the dope reaches is changed, and thereby thickness unevenness of the casting film occurs. Air enters between the casting film and the surface of the support in accordance with decrease in adhesion degree between the surface of the support and the casting bead. Accordingly, thickness unevenness of the film and defects on the surface of the film (surface undulation generated in the longitudinal and width directions of the film) occur. In view of the above, a film production apparatus as follows is disclosed in Japanese Patent Laid-Open Publication No. 2001-79864. In the film production apparatus, the clearance between the support and the decompression chamber is detected. When the clearance is less than a preset level, the decompression chamber is caused to move, to set the clearance between the support and the decompression chamber to the preset level or more. 
     Additionally, in a polymer film production method disclosed in Japanese Patent Laid-Open Publication No. 2002-103358, a wind shielding plate or fin as a wind shielding member is disposed at the vicinity of the casting die. In a cellulose ester film production apparatus disclosed in Japanese Patent Laid-Open Publication No. 2003-1655, the decompression chamber is provided with an adjustment plate as a labyrinth seal movable in a vertical direction, and in accordance with the vertical movement of the adjustment plate, the clearance between the adjustment plate and the surface of the support is adjusted. 
     However, when the solution casting method is performed continuously for long hours, the decompression chamber and the labyrinth seal drop by their own weight. Due to the dropping of the decompression chamber and the labyrinth, the clearance between the support and the labyrinth seal is changed, and thereby the pressure inside the decompression chamber is also changed. Thus, the quality of the film is decreased. Additionally, the distance between an original position of the labyrinth seal and a position of the labyrinth seal after the dropping varies with time, and therefore it is difficult to adjust the position of the labyrinth seal in consideration of the dropping. Accordingly, the time required for adjustment becomes longer and production efficiency is decreased. 
     Therefore, in the method for adjusting the clearance between the support and the decompression chamber at a preset level to prevent the pressure fluctuation inside the decompression chamber during the casting process, the working efficiency is poor, and there is a limit for producing a film efficiently. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide a casting device, and a solution casting method and apparatus capable of easily preventing pressure fluctuation inside a decompression chamber. 
     In order to achieve the above and other objects, a casting device of the present invention includes a support moving continuously, a casting die, and a decompression chamber. A casting die is used to discharge a casting dope onto the support to form a casting film. A decompression chamber is used to suck air of an upstream area from a casting bead in a moving direction of the support to decompress the upstream area. The casting bead is the casting dope extending from the casting die to the support. Two ledges are provided on the decompression chamber to form a labyrinth groove between the decompression chamber and the support. The labyrinth groove extends in a direction perpendicular to air flowing between the decompression chamber and the support. Each of the ledges includes an edge portion. The edge portion has a cross section with an acute angle in a direction of the flowing air. The flowing air occurs with the sucking. The labyrinth groove is formed between the edge portions. 
     Preferably, the edge portion is formed by a perpendicular surface perpendicular to the moving direction of the support and an inclined surface intersecting with the perpendicular surface such that the perpendicular surface and the inclined surface makes an acute angle. The labyrinth groove is preferably formed by the perpendicular surface and the inclined surface provided alternately in this order from an upstream side in the direction of the flowing air occurring with the suction. Preferably, the casting device further includes a shielding member disposed at both end portions in a longitudinal direction of the labyrinth groove. The shielding member is used for closing the labyrinth groove to shield the flowing air occurring with the suction. The support is preferably a drum rotated around a center of its cross section. The casting film is formed on a peripheral surface of the support. 
     A solution casting device of the present invention includes the casting device described above, and a drier for drying the casting film peeled from the support to form a film. 
     According to a solution casting method of the present invention, a casting dope is discharged from a casting die onto a support moving continuously to form a casting film. An upstream area from a casting bead in a moving direction of the support is sucked to decompress the upstream area by a decompression chamber. The casting bead is the casting dope extending from the casting die to the support. Air flowing between the decompression chamber and the support is compressed by one of two ledges for forming a labyrinth groove. The labyrinth groove extends in a direction perpendicular to the flowing air. The air occurs with the sucking. The compressed air is swollen by the labyrinth groove formed between edge portions. Each of the edge portions is provided at the ledges. The casting film is peeled from the support. The peeled casting film is dried to form a film. 
     According to the present invention, each of the pair of ledges for constituting the labyrinth groove includes an edge portion having a cross section with an acute angle. Therefore, the air flowing between the edge portion of the labyrinth groove and the support is compressed and further swollen in the labyrinth groove. As a result, it is possible to prevent the air from entering the decompression chamber. Accordingly, according to the present invention, it is possible to prevent pressure fluctuation inside the decompression chamber. Thus, it is possible to efficiently produce the film while preventing occurrence of thickness unevenness. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will be more apparent from the following detailed description of the preferred embodiments when read in connection with the accompanied drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
         FIG. 1  is an explanatory view schematically illustrating a film production line; 
         FIG. 2  is a side view schematically illustrating a casting die, a casting drum, and a decompression chamber; 
         FIG. 3  is an exploded perspective view schematically illustrating the decompression chamber; 
         FIG. 4  is a plan view schematically illustrating the decompression chamber viewed from a peripheral surface of the casting drum; 
         FIG. 5  is a cross sectional view taken along lines V-V of  FIG. 4 , schematically illustrating a lateral labyrinth plate and members at the vicinity of the lateral labyrinth plate according to a first embodiment; 
         FIG. 6  is a plan view of a portion surrounded by a chain double-dashed line VI of  FIG. 4 , schematically illustrating a labyrinth groove, viewed from the peripheral surface of the casting drum; 
         FIG. 7  is a cross sectional view schematically illustrating a lateral labyrinth plate according to a second embodiment; 
         FIG. 8  is a perspective view schematically illustrating a lateral labyrinth plate, a side labyrinth plate, and a shielding member according to a third embodiment; 
         FIG. 9  is a perspective view schematically illustrating a lateral labyrinth plate according to a fourth embodiment; 
         FIG. 10  is a cross sectional view schematically illustrating a lateral labyrinth plate according to a fifth embodiment; 
         FIG. 11  is a partial cross sectional view schematically illustrating a decompression chamber used in Examples; 
         FIG. 12  is a graph plotting a decompression degree P in the decompression chamber, an air flow velocity V sucked by a duct at the decompression degree P in Example 1; 
         FIG. 13  is a graph plotting a decompression degree P in the decompression chamber, an air flow velocity V sucked by a duct at the decompression degree P in Experiments 1 and 2 of Example 2; and 
         FIG. 14  is a graph plotting a decompression degree P in the decompression chamber, an air flow velocity V sucked by a duct at the decompression degree P in Experiment 3 of Example 2 and Experiment 3 of Example 1. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention are described in detail. However, the present invention is not limited thereto. 
     As shown in  FIG. 1 , a film production line  10  includes a stock tank  11 , a casting chamber  12 , a pin tenter  13 , a clip tenter  14 , a drying chamber  15 , a cooling chamber  16 , and a winding chamber  17 . 
     The stock tank  11  is provided with a stirrer blade  11   b  rotated by a motor  11   a  and a jacket  11   c.  Inside the stock tank  11  is stored a dope  21  as a raw material for a film  20 . A heat transfer medium flows inside the jacket  11   c  of the stock tank  11  such that a temperature of the dope  21  is adjusted to be within the range of 25° C. to 35° C. Since the stirrer blade  11   b  is rotated by the motor  11   a  in the stock tank  11 , it is possible to keep the dope  21  in a constant state while preventing aggregation of a polymer and the like. 
     A pump  25  and a filtration device  26  are disposed in a downstream side from the stock tank  11 . An adequate amount of the dope  21  is arbitrarily poured into the filtration device  26  from the stock tank  11  by use of the pump  25 , and filtered by the filtration device  26 . Thereby, impurities are removed from the dope  21 . 
     The casting chamber  12  includes a casting die  30 , a casting drum  32 , a peeling roller  34 , a temperature adjuster  35 , and a decompression chamber  36  so as to constitute a casting device. The casting die  30  is used as a means for discharging the dope  21 . The casting drum  32  is an endless support. The peeling roller  34  is used to peel a casting film  33  from the casting drum  32 . The temperature adjuster  35  adjusts the temperature inside the casting chamber  12 . The decompression chamber  36  is used as a decompression means. 
     As shown in  FIG. 2 , a discharge port  30   a  for discharging the dope  21  is provided at a front end of the casting die  30 . The dope  21  is cast through the discharge port  30   a  onto a peripheral surface  32   a  of the casting drum  32  disposed under the discharge port  30   a.  A material for the casting die  30  has high resistance to corrosion against an electrolyte aqueous solution, and a mixed liquid of methylene chloride, methanol, and the like. A coefficient of thermal expansion of the material for the casting die  30  is low. Accuracy of finishing of a contact surface of the casting die  30  to the liquid is preferably 1 μm or less in the surface roughness, and straightness thereof is preferably 1 μm/m or less in any direction. The casting die  30  as described above is used to form the casting film  33  having no streaks and thickness unevenness on the peripheral surface  32   a  of the casting drum  32 . 
     As shown in  FIGS. 1 and 2 , the casting drum  32  has an approximately cylindrical shape, and is rotated around a center of its cross section as a shaft by a not-shown driver. The not-shown driver causes the casting drum  32  to rotate such that the peripheral surface  32   a  of the casting drum  32  moves in a predetermined moving direction (hereinafter referred to as X direction) at a predetermined moving speed within the range of 10 to 300 m/min. The peripheral surface  32   a  of the casting drum  32  is subjected to chrome plating so as to have sufficient resistance to corrosion and strength. A heat transfer medium circulator  37  is attached to the casting drum  32 . The temperature of the heat transfer medium is kept at a desired value by the heat transfer medium circulator  37 . The heat transfer medium flows inside a heat transfer medium passage in the casting drum  32  such that a surface temperature of the casting drum  32  is kept within a desired range. 
     During a casting process, the dope  21  is discharged through the discharge port  30   a  onto the peripheral surface  32   a  of the casting drum  32  so as to form a casting bead  40  extending from the discharge port  30   a  to the peripheral surface  32   a.  The dope  21  is cast onto the moving peripheral surface  32   a  and spread thereon so as to form the casting film  33 . The casting film  33  is conveyed in the X direction at a predetermined speed in accordance with the rotation of the casting drum  32 . As described above, the dope  21  is continuously cast onto the moving peripheral surface  32   a  of the casting drum  32  so as to form the long casting film  33  on the peripheral surface  32   a.    
     The decompression chamber  36  is disposed in an upstream side from the casting die  30  in the X direction, and connected to a suction device  46  through a pipe  45 . The decompression chamber  36  sucks air of a cavity  60   a  of the decompression chamber  36  by the suction device  46 , and as a result, the upstream side from the casting bead  40  is decompressed such that the pressure in the upstream side from the casting bead  40  in the moving direction of the peripheral surface  32   a  is lower than that in the downstream side by 10 Pa to 1500 Pa. In accordance with the decompression, degree of adhesion between the peripheral surface  32   a  and the casting bead  40  is increased, and therefore it is possible to prevent air from entering between the casting film  33  and the peripheral surface  32   a.  The casting film  33  is cooled on the casting drum  32  so as to be hardened enough to have a self-supporting property. Thereafter, the casting film  33  is peeled from the casting drum  32  by use of the peeling roller  34  to be a wet film  47 . 
     As shown in  FIG. 1 , the temperature inside the casting chamber  12  is adjusted to be approximately constant within a predetermined range by the temperature adjuster  35 . The temperature inside the casting chamber  12  is preferably in the range of 10° C. to 30° C. Inside the casting chamber  12  is provided a condenser  48 . Outside the casting chamber  12  is provided a recovery device  49 . The solvent vapor in the casting chamber  12  is condensed into liquid by the condenser  48 , and further recovered by the recovery device  49 . The liquid is refined by a refining device to be reused as an organic solvent for preparing the dope. A condensation point of the solvent in the casting chamber  12  is kept within the range of −10° C. to 25° C. In a case where the condensation point of the solvent in the casting chamber  12  is less than −10° C., the solvent easily evaporates. Therefore, plate out easily occurs, unfavorably. Note that the plate out means precipitation of some undesired substances on the peripheral surface  32   a.  In contrast, in a case where the condensation point of the solvent in the casting chamber  12  exceeds 25° C., condensation of the solvent easily occurs on the peripheral surface  32   a.  The condensation of the solvent causes defect on the surface of the film, unfavorably. Note that, the condensation point means the temperature at which condensation of the solvent contained in the atmosphere starts. 
     The pin tenter  13  and the clip tenter  14  are disposed in the downstream side from the casting chamber  12 . In the pin tenter  13 , the wet film  47  is dried to be a film  20 . In the clip tenter  14 , the film  20  is stretched while being dried. In the pin tenter  13 , plural pins are inserted into the side ends of the wet film  47  and fixed thereto. While being conveyed in the pin tenter  13 , the wet film  47  is dried to be the film  20 . The film  20  still containing the solvent is sent to the clip tenter  14 . 
     In the clip tenter  14 , side ends of the film  20  are held by the plural clips moving continuously in accordance with the moving chain. Thereafter, while being conveyed in the clip tenter  14 , the film  20  is dried. The distance between the clips opposed to each other in the width direction of the film  20  is increased so as to apply tension to the width direction of the film  20 . Thereby, the film  20  is stretched. As described above, since the film  20  is stretched in the width direction, molecules in the film  20  are orientated, and thereby the film  20  comes to have optical properties such as retardation. Note that the clip tenter  14  may be omitted. 
     The side ends of the film  20  sent from the clip tenter  14  is cuts off by an edge slitting device  51 . The edge slitting device  51  is provided with a crusher  52 . After being cut away, the side ends of the film  20  are sent to the crusher  52  to be crushed into pieces. The pieces of film  20  thus crushed are reused as a primary dope. 
     The film  20  whose side ends are cut off by the edge slitting device  51  is sent to the drying chamber  15 . The drying chamber  15  includes plural rollers  53  and an adsorption and recovery device  54 . The film  20  is conveyed by the rollers  53  in the drying chamber  15 . The film  20  dried in the drying chamber  15  is sent to the cooling chamber  16  to be cooled therein such that the temperature of the film  20  goes down to at least 30° C. Then, the film  20  is sent to the winding chamber  17 . Additionally, a compulsory neutralization device (neutralization bar)  55  is disposed in the downstream side from the cooling chamber  16  that is next to the drying chamber  15 . Moreover, a knurling roller  56  is disposed in the downstream side from the neutralization device  55  in this embodiment. 
     The winding chamber  17  contains a winder  57  and a press roller  58 . The film  20  sent to the winding chamber  17  is wound around the core  57   a  rotated by the winder  57  while being pressed against the core  57   a  by the press roller  58 . 
     As shown in  FIGS. 2 and 3 , the decompression chamber  36  is constituted by a casing  60 . The casing  60  is formed by a pair of side boards  61  disposed along the X direction, a top board  62  bridged over the pair of side boards  61 , a first front board  63 , a second front board  64 , and a rear board  66  such that the inside of the casing  60  is the cavity  60   a.  Note that the casing  60  is disposed such that the lower end of each of the side boards  61  and the rear boards  66  is close to the peripheral surface  32   a.  The casing  60  has an opening  60   b  partially blocked by a front end  30   c  of the casting die  30  at its front side in the downstream side in the X direction. At the bottom of the casing  60  is provided an opening  60   c  so as to be close to the peripheral surface  32   a  of the casting drum  32 . Preferably, a material for the respective boards  61  to  66  is not easily dissolved into the organic solvent, and has strength enough to withstand differential pressure between the inside and the outside of the casing  60 . The respective boards  61  to  66  are made of stainless steel, for example. 
     As shown in  FIGS. 3 and 4 , plural plates  71  and  72  are disposed so as to stand upright along the X direction in the casing  60 . The plural plates  71  and  72  divide the cavity  60   a  of the casing  60  into plural sections in a width direction of the casting film  33  (hereinafter referred to as Y direction). The plates  71  and  72  function as flow regulating plates for air (wind)  400  flowing due to the suction of the decompression chamber  36 . Among the plural plates  71  and  72 , the plates  71  disposed in the upstream side from each end  40   a  of the casting bead  40  in the X direction are referred to as outer side seal plates  71 , and the plates  72  disposed between the pair of outer side seal plates  71  are referred to as inner side seal plates  72 . 
     A lateral seal plate  73  is disposed along the Y direction in the casing  60 . The lateral seal plate  73  is fixed to the ends of the inner side seal plates  72  in the upstream side in the X direction such that the inner side seal plates  72  stand upright. Each of the seal plates  71  to  73  is preferably made of, for example, MC nylon (registered trademark) or Teflon (registered trademark) that is not easily dissolved into the organic solvent. 
     A pair of side labyrinth plates  76  and a lateral labyrinth plate  77  are disposed outside the casing  60 . The pair of side labyrinth plates  76  are disposed along the side boards  61 . The lateral labyrinth plate  77  is disposed along the rear board  66 . Each of the labyrinth plates  76  and  77  is provided with a labyrinth groove described later. The labyrinth grooves can prevent the flowing air  400  from entering the cavity  60   a.  Note that in a case where the side labyrinth plates  76  and the lateral labyrinth plate  77  are not used, the labyrinth groove may be directly provided in a bottom surface of each of the side boards  61  and the rear board  66  constituting the casing  60 . Note that the lines V-V of  FIG. 4  correspond to the direction of flowing air in a case where the labyrinth groove is formed on the lateral labyrinth plate  77 . In contrast, the lines V-V of  FIG. 4  correspond to a direction perpendicular to the direction of flowing air in a case where the labyrinth groove is formed on the side labyrinth plates  76 . 
     As shown in  FIG. 5 , the lateral labyrinth plate  77  is fixed to an end portion  66   a  of the rear board  66  through a mounting bracket  83  with a screw  80  and a nut  81 . The lateral labyrinth plate  77  is disposed at a lower end portion of the end portion  66   a  and extends along the Y direction. The lateral labyrinth plate  77  is composed of five seal members  85  arranged so as to be in close contact with each other in the X direction. The seal member  85  is preferably made of MC nylon (registered trademark) and Teflon (registered trademark) that is not easily dissolved into the organic solvent. 
     As shown in  FIGS. 5 and 6 , each of the seal members  85  is disposed along the Y direction and vertical to the peripheral surface  32   a.  Each of the seal members  85  is disposed such that the end portion thereof protrudes from the lower end of the decompression chamber  36  toward the peripheral surface  32   a  of the support  32 . Each of the protruded portions, in other words, a ledge includes an end portion  85   a  having a groove forming portion  86  extending along the Y direction. Note that in a case where the labyrinth groove is directly formed on the bottom surface of each of the side boards  61  and the rear board  66  constituting the casing  60 , each of the side boards  61  and the rear board  66  may include a ledge similar to the above on its bottom surface. 
     The groove forming portion  86  is composed of a bottom surface  86   a,  an inclined surface  86   b,  an edge portion  86   c,  and a vertical surface  86   d  in this order from the downstream side to the upstream side in the X direction. A clearance between the bottom surface  86   a  and the peripheral surface  32   a  is approximately constant along the X and Y directions. A clearance between the inclined surface  86   b  and the peripheral surface  32   a  is gradually decreased from the downstream side to the upstream side in the X direction. The edge portion  86   c  is defined by the inclined surface  86   b  and the vertical surface  86   d  in the upstream side from the inclined surface  86   b  in the X direction. Each of the edge portions  86   c  has a cross section with an acute tip angle θ 1  in a direction of the flowing air. The tip angle θ 1  is preferably in the range of 20° to 60°, and more preferably in the range of 30° to 50°. An area of the cross section of the groove forming portion  86 , which is perpendicular to the Y direction, is preferably in the range of 300 to 2000 mm 2 , and more preferably in the range of 700 to 1500 mm 2 . Note that the groove forming portion  86  may be disposed such that the vertical surface  86   d  and the peripheral surface  32   a  intersect with each other at an acute angle and the inclined surface  86   b  and the peripheral surface  32   a  intersect with each other at a right angle. 
     The seal members  85  each having the groove forming portion  86  at its end portion  85   a  are arranged so as to be in close contact with each other in the X direction, and thereby labyrinth grooves  87  are formed along the Y direction at the lower end portion of the lateral labyrinth plate  77 . 
     Next, an operation of the film production line  10  having the above-described structure is described. As shown in  FIGS. 1 and 2 , the casting drum  32  rotates around the shaft such that the peripheral surface  32   a  thereof moves in the X direction. The dope  21  is cast through the discharge port  30   a  onto the peripheral surface  32   a  to form the casting bead  40  extending from the discharge port  30   a  to the peripheral surface  32   a.  The suction device  46  sucks air of the cavity  60   a  of the decompression chamber  36 . Due to the suction, the air in the upstream side from the casting bead  40  flows toward the cavity  60   a.    
     Upon movement of the peripheral surface  32   a,  the flowing air  400  is generated along the peripheral surface  32   a  so as to flow toward the casting bead  40 . Due to the suction by the suction device  46 , the flowing air  400  is flown into the opening  60   c  through the clearance between the lateral labyrinth plate  77  and the peripheral surface  32   a.    
     As shown in  FIG. 5 , according to the present invention, the labyrinth grooves  87  are formed at the end portion of the lateral labyrinth plate  77  in the periphery of the peripheral surface  32   a.  The labyrinth grooves  87  are constituted by the seal members  85  each having the edge portion  86   c.  Each of the edge portions  86   c  has a cross section with an acute angle in a direction of the flowing air. Therefore, the flowing air  400  frown into the clearance between the lateral labyrinth plate  77  and the peripheral surface  32   a  is compressed at the time of passing through the clearance between the edge portion  86   c  and the peripheral surface  32   a,  and further swollen in the labyrinth groove  87  constituted by the bottom surface  86   a  and the inclined surface  86   b.  Since the flowing air  400  is compressed and swollen as described above, it is possible to prevent the flowing air  400  from entering through the opening  60   c.  Further, according to the present invention, since it is possible to increase airtightness of the decompression chamber  36 , even when the clearance between the decompression chamber  36  and the peripheral surface  32   a  changes, it is possible to prevent pressure fluctuation inside the decompression chamber  36  caused by the change in the clearance. Therefore, according to the present invention, it is possible to prevent pressure fluctuation of the cavity  60   a  caused by the flowing air  400  entering through the opening  60   c  during the casting process. Therefore, it is possible to produce the film while preventing occurrence of thickness unevenness and the defect on the surface of the film. 
     The edge portion  86   c  may have any shape as long as it can compress the air passing through the clearance between the edge portion  86   c  and the peripheral surface  32   a.  Each of the inclined surface  86   b,  the bottom surface  86   a,  and the vertical surface  86   d  of the labyrinth groove  87  may have any shape as long as the flowing air  400  passing through the clearance between the edge portion  86   c  and the peripheral surface  32   a  can be swollen in the labyrinth groove  87 , and preferably the inclined surface  86   b  has a shape allowing the air just after having passed through the clearance between the edge portion  86   c  and the peripheral surface  32   a  to be swollen. A depth D of the labyrinth groove  87 , which is obtained by subtracting a seal clearance G from a clearance between the bottom surface  86   a  and the peripheral surface  32   a,  is preferably gradually increased toward the opening  60   c.    
     The lateral labyrinth plate  77  is preferably attached to the decompression chamber  36  such that the seal clearance G between the edge portion  86   c  and the peripheral surface  32   a  is within the range of 0.1 to 5 mm. Further the seal clearance G is preferably within the range of 0.3 to 2 mm. In a case where the lateral labyrinth plate  77  has plural edge portions  86   c,  the smallest clearance between the edge portion  86   c  and the peripheral surface  32   a  may be considered as the seal clearance G. A thickness t 1  of the seal member  85  is preferably within the range of 1 to 20 mm. Further, it is preferable that a width ta of the bottom surface  86   a  in the X direction is within the range of 1 to 20 mm, a width tb of the inclined surface  86   b  in the X direction is within the range of 0.1 to 1 mm, and the depth D of the labyrinth groove  87  is within the range of 1 to 10 mm. 
     Although the end portion  85   a  has the bottom surface  86   a,  the inclined surface  86   b,  the edge portion  86   c,  and the vertical surface  86   d  in this order from the downstream side to the upstream side in the X direction in the above embodiment, the present invention is not limited thereto. The order may be from the upstream side to the downstream side in the X direction. 
     Although the end portion  85   a  of the seal member  85  is provided with the groove forming portion  86  including the bottom surface  86   a,  the inclined surface  86   b,  the edge portion  86   c,  and the vertical surface  86   d  in the above embodiment, the present invention is not limited thereto. Alternatively, as shown in  FIG. 7 , the end portion  85   a  of the seal member  85  may be provided with the groove forming portion  86  including the inclined surface  86   b,  the edge portion  86   c,  and the vertical surface  86   d.  Note that, as long as the edge portion  86   c  has a cross section with an acute angle, the cross section of the labyrinth groove  87  may be any shape such as a V-shaped groove, a U-shaped groove, and a square groove. 
     Although the lateral labyrinth plate  77  is composed of the five seal members  85  arranged so as to be in close contact with each other in the X direction in the above embodiment, the present invention is not limited thereto. The lateral labyrinth plate  77  may be composed of at least two seal members  85  arranged so as to be in close contact with each other in the X direction and thereby has the labyrinth grooves  87 . Note that it is also possible to form the labyrinth grooves  87  at the end portion of the lateral labyrinth plate  77  by machining or the like, instead of arranging the seal members  85  in a close contact manner so as to form the labyrinth grooves  87 . In this case, the end portion of the lateral labyrinth plate  77  facing toward the support  32 , namely the lower end portion of the lateral labyrinth plate  77 , may include a ledge as described above. 
     Although the labyrinth groove  87  is formed at the end portion of the side labyrinth plate  76  and the lateral labyrinth plate  77  in the above embodiment, the present invention is not limited thereto. The labyrinth groove  87  may be formed at least one of the side labyrinth plate  76  and the lateral labyrinth plate  77 . 
     The labyrinth groove  87  is preferably formed at the end portion of each of the outer side seal plate  71 , the inner side seal plate  72 , and the lateral seal plate  73 , in addition to the side labyrinth plate  76  and the lateral labyrinth plate  77 . Thereby, it becomes possible to increase flow regulating effect in the periphery of side edges of the casting bead  40  and prevent vibration of the casting bead  40 . 
     As shown in  FIG. 8 , in a case where each of the labyrinth grooves  87  is provided in an entire area in the Y direction of the lateral labyrinth plate  77 , a shielding member may be provided at the ends of the lateral labyrinth plate  77  in the Y direction so as to shield the cross sections of each of the labyrinth grooves  87 . Although the shielding member is not especially limited as long as it can shield the cross sections of each of the labyrinth grooves  87 , the side labyrinth plate  76  or the like may be used as the shielding member. As shown in  FIG. 8 , for example, the side labyrinth plate  76  may be disposed so as to shield the ends in the Y direction of the lateral labyrinth plate  77 . Alternatively, in a case where each of the labyrinth grooves  87  is provided in an entire area in the X direction of each of the side labyrinth plates  76 , for example, a shielding member  88  may be disposed at the end in the X direction of the labyrinth groove  87 . Note that the shielding member  88  may be integrated with the seal member  85 . 
     Further, as shown in  FIG. 9 , the shielding member  88  may be provided with an inclined surface  88   b,  an edge portion  88   c,  and a vertical surface  88   d  in this order from the cavity  60   a  toward the outside of the decompression chamber  36 . The inclined surface  88   b  has the same shape as that of the inclined surface  86   b.  The vertical surface  88   d  has the same shape as that of the vertical surface  86   d.  As in the case of the edge portion  86   c,  the edge portion  88   c  preferably has a cross section with an acute angle in a direction of the flowing air. Alternatively, the shielding members  88  may be aligned in the Y direction. 
     As the width of the film  22  to be produced is increased, the width of the casting film is also increased. As a result, there easily occurs pressure fluctuation of the cavity  60   a  of the decompression chamber  36 . According to the casting device of the present invention, even if the width of the casting film is increased, it is possible to prevent the pressure fluctuation of the cavity  60   a  of the decompression chamber  36 . The width of the casting film is preferably at least 600 mm, and more preferably in the range of 1400 to 2500 mm, for example. Additionally, in a case where the width of the casting film is more than 2500 mm, the present invention is effective. 
     According to the present invention, as long as the tip angle θ 1  made between the inclined surface  86   b  and the vertical surface  86   d  is acute, the tip angle θ 1  of the edge portion  86   c  is acute. Accordingly, the present invention is not limited to the above embodiments, and a lateral labyrinth plate  91  shown in  FIG. 10  is also applicable in the present invention. The labyrinth plate  91  is composed of seal members  90  arranged so as to be in close contact with each other in the X direction. A groove forming portion  96  is formed at an end portion of each of the seal members  90  in the periphery of the peripheral surface  32   a.  The groove forming portion  96  is composed of a bottom surface  96   a,  an inclined surface  96   b,  an edge surface  96   e,  and a vertical surface  96   d  in this order from the cavity  60   a  toward the outside of the decompression chamber  36 . The bottom surface  96   a  has the same shape as that of the bottom surface  86   a.  The inclined surface  96   b  has the same shape as that of the inclined surface  86   b.  The vertical surface  96   d  has the same shape as that of the vertical surface  86   d.  As long as the tip angle θ 1  made between the inclined surface  96   b  and the vertical surface  96   d  is acute, an embodiment in which the edge surface  96   e  is provided instead of the edge portion  86   c  is naturally also applicable. The width te of the edge surface  96   e  in the X direction is preferably 1.5 mm, and more preferably at most 1.0 mm. The seal members  90  each having the groove forming portion  96  at its end portion are arranged so as to be in close contact with each other in the X direction, and thereby labyrinth grooves  97  are formed along the Y direction at the end portion of the lateral labyrinth plate  91  in the periphery of the peripheral surface  32   a.    
     Further, for the purpose of casting the dope, co-casting by simultaneous stacking and co-casting by sequential stacking can be selectively used. In the co-casting by simultaneous stacking, two or more kinds of dopes are subjected to co-casting simultaneously to be stacked. In the co-casting by sequential stacking, plural kinds of dopes are subjected to co-casting sequentially to be stacked. Note that the co-casting by simultaneous stacking and the co-casting by sequential stacking may be combined to be used. In the co-casting by simultaneous stacking, a casting die provided with a feed block may be used, or a multi-manifold-type casting die may be used. Note that, in a multilayer film obtained by the co-casting, at least any one of thickness of the layer at the side exposed to air and the thickness of the layer at the side of the support is preferably 0.5 to 30% relative to the total thickness of the film. Further, in the co-casting by simultaneous stacking, when the dope is cast onto the support through a die slit (discharge port), the dope with high viscosity is preferably surrounded by the dope with low viscosity. In the casting bead formed so as to extend from the die slit to the support, the dope exposed outside preferably has a relative proportion of alcohol higher than that of the dope located inside. 
     Moreover, the present invention is also applicable to a casting device using a casting belt instead of the casting drum  32 . The casting belt is bridged over rotation rollers and moves. 
     EXAMPLE 1 
     (Experiment 1) 
     In Experiment 1, a decompression chamber  100  shown in  FIG. 11  was used. The decompression chamber  100  is composed of a casing  101  and the lateral labyrinth plate  77 . The casing  101  is a box and disposed above the support  102 . The casing  101  is composed of a top board, a pair of side boards, and a front board. Each of the bottom and rear of the casing  101  has an opening, and a cavity  101   a  is exposed outside through each of the openings. The lateral labyrinth plate  77  is disposed on the rear side of the casing  101 , which has the opening, so as to close the opening. The pair of side boards and the front board are disposed so as to face the support  102 . Accordingly, the cavity  101   a  is in an approximately hermetically-sealed. The lateral labyrinth plate  77  is composed of four seal members  85  arranged so as to be in close contact with each other in the X direction. Thereby, three labyrinth grooves  87  shown in  FIG. 5  are formed. Note that for the purpose of preventing complexity of the drawing, the labyrinth groove  87  is not shown in detail in  FIG. 11  (Refer to  FIG. 5 ). The width ta of the bottom surface  86   a  of the labyrinth groove  87  was 3 mm, the width tb of the inclined surface  86   b  was 5 mm, and the depth D of the labyrinth groove  87  was 8.65 mm. As shown in  FIG. 11 , a position of the lateral labyrinth plate  77  was adjusted such that the seal clearance G was in the range of 0.3 to 2 mm. The pipe  45  connects the casing  101  and the suction device  46  (see  FIG. 1 ). A not-shown air flow velocity meter (Climomaster made by KANOMAX JAPAN, INC.) and a not-shown probe (MODEL 6552) are disposed in the pipe  45 . The air flow velocity meter and the probe are used to detect air flow velocity V sucked by the duct of the pipe  45  (hereinafter referred to as duct sucking air flow velocity V). The suction device  46  sucks air of the cavity  101   a  such that the cavity  101   a  is decompressed to have a predetermined decompression degree P. The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked. 
     (Experiment 2) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 1 except that the lateral labyrinth plate  91  composed of the seal members  90  shown in  FIG. 10  was disposed instead of the lateral labyrinth plate  77  and the width te of the edge surface  96   e  was 1 mm. 
     (Experiment 3) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 1 except that the lateral labyrinth plate was composed of the five seal members  85  arranged so as to be in close contact with each other. 
     (Experiment 4) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 1 except that a seal member having thickness of 5 mm and no groove forming portion  86  at its end portion was used instead of the lateral labyrinth plate  77 . 
     The duct sucking air flow velocity V (unit;m/s) measured at the predetermined decompression degree P in each of the Experiments 1 to 4 is shown in  FIG. 12 . The data in Experiment 1 is denoted by “o”, the data in Experiment 2 is denoted by “□”, the data in Experiment 3 is denoted by “Δ”, and the data in Experiment 4 is denoted by “x”. 
     Embodiment 2 
     (Experiment 1) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 1 of Example 1 except that the position of the lateral labyrinth plate  77  was adjusted such that the seal clearance G was half of that in Experiment 1 of Example 1. 
     (Experiment 2) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 2 of Example 1 except that the position of the lateral labyrinth plate  91  was adjusted such that the seal clearance G was half of that in Experiment 2 of Example 1. 
     (Experiment 3) 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P was checked under the same conditions as those in Experiment 2 of Example 1 except that the position of the seal member was adjusted such that the seal clearance G was half of that in Experiment 4 of Example 1. 
     The duct sucking air flow velocity V measured at the predetermined decompression degree P in each of the Experiments 1 and 2 is shown in  FIG. 13 . The data in Experiment 1 is denoted by “o”, and the data in Experiment 2 is denoted by “□”. Further, the duct sucking air flow velocity V measured at the predetermined decompression degree P in each of Experiment 3 of Example 2 and Experiment 3 of Example 1 is shown in  FIG. 14 . The data in Experiment 3 of Example 2 is denoted by “×”, and the data in Experiment 3 of Example 1 is denoted by “Δ”. 
     By referring to  FIGS. 12 and 13 , it is possible to prevent air from flowing into the cavity  60   a  from outside of the decompression chamber  36  in the present invention. Therefore, according to the present invention, it is possible to prevent pressure fluctuation of the cavity  60   a  caused by the air flowing into the cavity  60   a.  Further, it becomes possible to prevent occurrence of thickness unevenness. Additionally, by referring to  FIG. 14 , even when the seal clearance G is increased, the seal member of the present invention makes it possible to achieve the duct sucking air flow velocity V which is the same as that obtained by using a conventional seal member. When the seal clearance G is changed, the duct sucking air flow velocity V is also changed in accordance with the change in the seal clearance. Namely, the change amount of the duct sucking air flow velocity V is increased as the seal clearance G is decreased. Further, when the seal clearance G is decreased, scratches may be generated on the surface of the support in some cases, and thereby resulting in unfavorable result. Accordingly, according to the present invention, it is possible to prevent the increase in the duct sucking air flow velocity V without causing scratches on the surface of the support and without adjusting the seal clearance G with high precision. 
     Various changes and modifications are possible in the present invention and may be understood to be within the present invention.