Process for the manufacture of selfsupporting tubes of thermoplastic material

Claim 1

IN A PROCESS FOR THE MANUFACTURRE OF SELF-SUPPORTING TUBES OF THICKNESS OF 0.005 TO 0.30 INCH OF THERMOPLASTIC MATERIAL HAVING A BASIS OF A PLASTICISED CELLULOSE ESTER OF A PARAFINIC MONOCARBOXYLIC ACID CONTAINING 2 TO 4 CARBON ATOMS BY EXTRUDING A TUBE OF THE MATERIAL HOT IN THE ABSENCE OF VOLATILE LIQUIDS, PROGRESSIVELY COOLING IT BY MEANS OF AIR JETS TO A TEMPERATURE AT WHICH IT IS NO LONGER PLASTIC, WHILE DRAWING IT AWAY FROM THE EXTRUSION POINT BY TRACTION APPLIED AT A POINT AT WHICH IT HAS CEASED TO BE PLASTIC, THE STEPS OF CONTROLLING THE CROSS-SECTIONAL SHAPE OF THE TUBE BY DRAWING IT BETWEEN THE EXTRUSION POINT AND THE TRACTION POINT THROUGH AN APERTURE BETWEEN A PAIR OF CONTACTING, FREELY ROTATABLE PROFILED ROLLS HAVING THEIR AXES PERPENDICULAR TO THE DIRECTION OF TRAVEL OF THE TUBE AND CONTROLLING THE TEMPERATURE OF THE TUBE SO THAT ON REACHING SAID APERTURE IT IS SOFT ENOUGH TO BE DEFORMED, BUT IS NOT LIMP, AND COOLING SAID TUBE IN SAID APERTURE SO THAT ON LEAVING THE APERTURE SAID TUBE IS HARD ENOUGH TO RETAIN THE CROSS-SECTIONAL SHAPE CONFERRED ON IT BY THE SAID ROLLS, WHILE MAINTAINING THE INNER SURFACE OF THE TUBE OUT OF CONTACT WITH ANY SUPPORTING SURFACE DURING THE TRAVEL OF SAID TUBE FROM SAID EXTRUSION POINT TO THE POINT WHERE SAID TUBE LEAVES SAID APERTURE.