the strands 10 are blown by a high velocity fluid to result in a tubular preform 10 u2032 constituted by a matrix of entangled strands 10 unlike the previously described prior art after the preform 10 u2032 exits the forming apparatus 200 the preform 10 u2032 travels through the pinch device 300 where it is pinched at intervals and is formed with necked down sections 11 u2032 along the length of the preform 10 u2032 the diameter of the preform 10 u2032 is thus reduced at the sections 11 u2032 to such a degree to make the preform 10 u2032 flexible at the necked down sections 11 u2032 after cooling and setting referring to fig3 the pinch device 300 comprises a pair of movable die members 310 320 each including a driving rod 330 340 the opposing front edges of the die members 310 320 are arcuately contoured to act as pinching faces 310 u2032 320 u2032 during each stroke of the pinch device 300 the movable die members 310 320 reciprocate to move toward and away from each other between an open position shown in fig4 and a closed position shown in fig5 in the open position the pinching faces 310 u2032 320 u2032 are spaced apart from each other with a gap wider than the diameter of the preform 10 u2032 while in the closed position the die members 310 320 overlap each other at the opposing edges thereof in this way reciprocation of the die members 310 320 squeezes the softened wall of the tubular preform 10 u2032 and forms one necked down section 11.