a continuous method of extruding an oriented plastic article of improved strength . a chemically reactive polymer , monomer or other compound is included in the matrix material and a parison is extruded or melt formed having built - in thermoplastic orientation caused by shear or caused by elongational flow . a reaction can be activated in the molten state thereof by additional heat resulting in a cross - linked structure or at least the mobility of the molecules being reduced , resulting in a longer relaxation time which makes the freezing of permanent orientation possible . the soft matrix can be stretched while still hot in the hoop and / or axial direction . then , the product thus obtain is calibrated and cooled in an oriented state .