| 摘要 |
PURPOSE:To gently reduce the thickness of a film from the thick part thereof, to enhance the strength and to take two films at one time, by using a die having a ring-shaped lip having a constant width and locally cooling the film in the vicinity of a frost line at the time of extrusion molding. CONSTITUTION:Molten plastic is extruded from the lip 1a of the die 7 by in extruder 2 and the extrudate is cooled by an air ring 5 and a cooling cylinder 6. At the same time, cooling air for local cooling guided by respective fins 8 is sprayed to the local cooling region (e) long in an axial direction held between the shield plates 24, 24 of the local cooling apparatus 8 on the outer peripheral surface of a bubble 3. Therefore, the solidification, that is, crystallization of the molten plastic passing through the local cooling region (e) is rapidly advanced as compared with the part other than the local cooling region (e) of the bubble 3 in the vicinity of a frost line. As a result, the part of a non-solidified state of the bubble 3 is stretched, but the stretching action in the axial and radius directions does not reach a thick wall part. Therefore, a film having an offset wall part 3a becoming thick in the outer peripheral cross-section in the axial direction opposed to the local cooling region (e) as compared with other part can be prepared. |
