A sack is constructed from a thermoplastic film of which the side portions are brought into partial overlapping relationship to define a tube section and top and bottom weld seams are applied transversely. A longitudinal weld seam applied within the confines of the region of overlap terminates at a spacing...http://www.google.es/patents/US4524460?utm_source=gb-gplus-sharePatente US4524460 - Valved sack
