Extrusion of thermoplastic tubes

Claim 1

THE METHOD OF FABRICATING CONTINUOUS, SELF-SUPPORTING, TUBULAR THERMOPLASTIC ARTICLES WHICH COMPRISES PASSING MOLTEN THERMOPLASTIC MATERIAL THROUGH A FIRST ANNULAR DIE, ACROSS A THERMAL BARRIER AND THROUGH A SECOND ANNULAR DIE OF GREATER CROSS-SECTIONAL AREA THAN THAT OF SAID FIRST ANNULAR DIE; CONTACTING THE INTERNAL SURFACE OF THE EXTRUDED TUBE WITH A SUPPORT MANDREL TO REDUCE THE LINEAR SPEED OF THE EXTRUDED TUBULAR THERMOPLASTIC MATERIAL EMERGING FROM SAID SECOND ANNULAR DIE SO THAT THE EXTRUDED TUBE IS FORCED TO ASSUME THE SHAPE OF THE SECOND DIE THEREBY FORMING A LIQUID-TIGHT SEAL BETWEEN THE EXTRUDED TUBE AND THE SECOND DIE; CONTINUOUSLY CONTACTING THE INTERNAL SURFACE OF THE EXTRUDED TUBE WITH SID MANDREL UNTIL THE EXTENDED TUBE HAS BECOME SELF-SUPPORTING; PASSING COOLING LIQUID THROUGH THE MANDREL IN INDIRECT HEAT EXCHANGE WITH THE EXTRUDED TUBE AND THEN INTO DIRECT CONTACT WITH THE INTERNAL SURFACE OF THE TUBE; CONTACTING THE EXTERIOR OF THE EXTRUDED TUBE WITH COOLING LIQUID IMMEDIATELY AS IT EMERGES FROM SAID SECOND DIE; MAINTAINING THE INTERIOR AND EXTERIOR SURFACES OF SAID TUBE IN CONTACT WITH COOLING LIQUID UNTIL SAID TUBE IS COOLED TO BELOW ITS SOFTENING POINT; AND RECOVERING SAID TUBE AS SAID TUBULAR ARTICLE.