| 摘要 |
PURPOSE:To facilitate the removal of a core by a method wherein the outer diameter of the core, which inserts in a neck part, is made smaller than the inner diameter of the neck pat before crystallization by the shrinkage allowance of the inner diameter during crystallization. CONSTITUTION:The outer peripheral surface 4a, of a core 4a is true circle in cross-section and has a shape nearly corresponding to the inner peripheral surface 2a4, which is slightly convergingly tapered downwards, of a neck part 2a. In addition, the outer diameter of the core 4a is made smaller than the corresponding inner diameter of the neck part 2a before crystallization at the corresponding portion nearly by the shrinkage allowance of said inner diameter after heating for crystallization by such a degree of amount as to facilitate the removal of the core 4a from the neck part 2a after crystallization. In other words, the clearance (d) between the core 4a and the neck part 2a before crystallization is about half said shrinkage allowance. In this case, no irregular deformation develops in the neck part 2a at crystallization and the inner peripheral surface remains to be true circle in cross-section and further the favorable result of the easy removal of the core 4 is obtained. |
