| 摘要 |
PURPOSE:To protect edge portion from breakdown and to lengthen service life of tube by finely splitting the edge portion of a flap and forming many small sections. CONSTITUTION:An edge portion 6 of a flap 4 is splitted finely to from many small sections 6a. Consequently, rigidity at the edge portion 6 lowers thereby preventing local concentration of stress in a tube. In order to from finely splitted edge portion 6 in the flap 4, strip knurls 14 are formed in a portion for forming the edge portion which faces an outer circumferential face mold 13 for inner circumferential face metal molds 11, 12, thereafter the edge portion 6 is molded. With such arrangement, small section can be molded at the edge portion with high productivity thus lengthening service life of the tube.
|
