BOOT

PURPOSE:To give a thick part corresponding to a pitch variation to each part of bellows structure, by varying a pitch between crest parts of the bellows structure whose crest and root parts have a constant diameter. CONSTITUTION:Pitches a, b,...h, and i between crest parts 1-1', 1'-1''... of a bellows structure are varied on the whole and be degrees. In this case, on the assumption that there is no draw-down in time of hollow molding, thickness becomes thin at the small side (a side) in the pitch but it becomes thick at the large side (i side) in the pitch. Therefore, in case of operating a boot, for example, when the boot is contracted, this contraction starts from the thinner side, and conversely when the boot is expanded wide, this expansion also starts from the thinner side. Even in the case where the boot is bent, no only operations for expansion or contraction, also this bending starts from the thinner side, yet larger bending takes place.