| 摘要 |
The valve tongue (11) is connected to an inwardly folded corner flap (5) which together with side flaps (6,7) folded over it defines a valve aperture. The tongue extends further inwards than the cover reinforcement sheet (13). The inner edge area of the sheet is not coated with adhesive against the folded-in side flaps, so that a check valve effect is obtained. The tongue comprises pref. two sheets (21,22) lying one on top of the other, having different degrees of flexibility. The lower sheet (21) is stiffer tban the upper (22), which is facing the valve aperture. The lower sheet is suitably of paper, and the other of plastic film. |
