| 摘要 |
A straight inner pipe(2) and a straight outer pipe(3) are brought together as a double pipe(1), and protrusions(4) are hydraulically formed on the double pipe. Next, the outer pipe, through the formation of an air gap(8) between the inner and outer pipes, is widened, and the protrusions are then opened out into inlet connections on their end face sides. The double pipe is bent before widening of the outer pipe and this is carried out by internal hydraulic pressure.
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