| 摘要 |
<p>Valved plastic sack has air vents produced by perforating part of the plastic film by heating needles up to the plasticising temp. or higher whilst forming a projecting edge around the openings. Specif. the openings are made in the bottom valve section above the valve, and pref. the openings in the valve flap are offset from those in the bottom valve flap.</p> |
