A plastic sack for containing a liquid and used for medical purposes is formed from two plastic (e.g. polyvinyl chloride) sheets welded together around their edges. The inner surface of one (pref. both) of the sheets is formed with ribs which maintian the sheets separated. The ribs may be formed by deforming each sheet or by solid integral projections on the surface of each sheet. The ribs may extend along or across the sack or may be arranged in a herring-bone pattern. The two sheets do not have to be pulled apart manually before the sack can be filled with liquid.
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