| 摘要 |
PURPOSE: Molding failure in a convex reinforcing portion by air expansion is prevented having arch type cross section, thereby guaranteeing strong durability and support for earth pressure. CONSTITUTION: An extraction tube(4) is extracted from a dice(3) along a cooling pipe(17). Tubes(9,11) extracted from other dice(7,8) are formed in a molder(1) spirally maintaining a certain interval. A hollow portion(31) is formed by spirally forming the tubes. An attaching portion is integrally formed spirally with the hollow portion. A convex reinforcing portion(13) is formed from the upper side of the hollow portion to the attaching portion.
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