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On the basis of ofientation techniques of plastic pipes domenstic and abroad.a biaxial oriention mechanism for self-oreinted pipes was developed from the hop-pocket expantion technique of the heat shrinkable tubes.The hop-pocket expantion technique provids a new solusion of continuous expansion of plastics pipes in solid state in procccs of manufacturing biaxial oriented pipes.Experiments shows the expanded pipes have uniform appearance with larger diameter and considerably higher tensile suength and elastic modulus.