| 摘要 |
A manufacturing method of a double spiral glass tube having a bump that is difficult to break. The manufacturing method includes the steps of: causing the bump forming section (32) in the central portion (13) to have greater tube thickness than the other sections in the glass tube (23) that has been deformed into a double spiral shape; heating and softening part of the bump forming section; and forming a bump (14) by injecting a nitrogen gas (27) into the inner space of the glass tube from both ends thereof to expand the bump forming section. With this structure, the bump forming section is preliminarily formed to have a greater tube thickness than the other sections. As a result, the bump, which is formed by expanding the bump forming section to have a smaller tube thickness than before the expansion, still maintains a sufficient strength.
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