| 摘要 |
According to a method for producing a duct, a cylindrical preform, which opens at its both end portions in a longitudinal direction and has a central part, is formed. The central part is narrowed with respect to the both end portions. The central part has a smaller cross-sectional area than that of the both end portion. The preform is formed from a thermoplastic material, which increases in strength when stretched. Next, the preform is heated. Then, stretch blow molding is performed on the preform by stretching the preform in axial and radial directions thereof, such that the cross-sectional area of the central part becomes generally the same as that of the both end portion, or equal to or larger than that of the both end portion. The central part is narrowed to such an extent that it retains predetermined strength after performing the molding on the preform.
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