a hand - holdable , portable welding apparatus for smoothly joining the lumens of sections of thermoplastic tubing systems , including those that may be located in a cramped work site . the integrity of the lumens is maintained when the tubes are being welded by use of a selectively rigidizable , non - elastomeric mandrel of ptfe or pfa having a predetermined maximum size dimensioned to the id of the lumens being joined . the welding unit employs a resiliently mounted coaxial heater block in conjunction with arcuate radiator inserts that are centered about the tubes to be welded regardless of the diameter of the tubes . spring - loaded dogs grab the periphery of the tubes as they are inserted into the welding unit and prevent them from moving apart when heat is applied .