The air exhaust thru-holes are formed, for example, in round corner portions 10 i and 11 i so that indentation thereof may not remain in the resulting formed part. [0102]FIGS. 12A, 12B, and 12C are enlarged diagrams of a portion C enclosed with a dotted line in FIG. 11, of which FIG. 12A is a diagram for explaining a fluid sealing