in the apparatus for preparing heat - shrinkable plastic tubing by radially expanding lengths of plastic tubing to a larger diameter within a rigid cylinder by heating the cylinder externally and pressurizing the interior of the extruded plastic tubing with gas to cause the tubing to radially expand against the interior walls of the rigid cylinder , the improvements which comprise : a . employing in place of the rigid cylinder a heat - resistant flexible sheet looped into a cylindrical shape to form a restraint , b . employing a pair of clamps extending longitudinally along the cylindrical shaped sheet along the portions of the sheet which meet and abut to form the cylinder , positioned such that the ends of said sheet are between the clamps , and c . employing means for heating the tubing comprising a device for injecting a hot gas into the interior of the extruded plastic tubing .