then closed during a period of 20 seconds the tubing was further heated by the steam and expanded so that it pressed against the flexible restraining tube a valve was then opened to admit air at 30 psig the steam valve was simultaneously closed the exit valve was then opened slightly to permit gas to escape but to maintain the pressure inside the fep plastic tubing compressed air was allowed to bleed through the expanded tubing until the tubing had cooled sufficiently so that the pressure could be removed without the tubing shrinking the result was a piece of plastic tubing with a cylindrically expanded section two end sections of the original unexpanded diameter and two transition sections between the end diameter and the expanded diameter cutting off the end and transition sections gave a length of heat shrinkable plastic tubing which had the desired property of shrinking back to its original diameter when heated above 150 u00b0 c the heat shrinkable plastic tubing was smooth with a circular cross section except that there was a visible line where the ends of the restraining loop had been clamped together this line or seam was raised enough that it could be felt by passing a finger over it the process of example 1 was substantially repeated except the unexpanded plastic tubing was 11 2 inches in diameter the expanded tubing 2 inches in diameter and the restraining flexible sheet cylinder was made of 10 mil du pon armalon tfe fluorocarbon resin coated glass fabric the unexpanded tube had the following thickness at various points 24 26 25 23 mils the expanded tubing had the following thickness.