Process for the production of tubular films from thermoplastic materials

Claim 1

A PROCESS FOR THE PRODUCTION OF AN ORIENTED TUBULAR FILM COMPRISING DOWNWARDLY MELT EXTRUDING A TUBE OF AN ORGANIC THERMOPLASTIC POLYMERIC MATERIAL HAVING A GLASS TRANSITION TEMPERATURE OF AT LEAST 25*C FROM AN EXTRUSION ORIFICE, COOLING AND SIZING THE TUBE BY WITHDRAWING THE TUBE OVER AND IN CONTACT WITH A HOLLOW, COOLED, MATT- SURFACED MANDREL AT A RATE OF FROM 1.05 TO 10 TIMES GREATER THAN THE SPEED OF EXTRUSION OF THE TUBE SO AS TO STRETCH THE TUBE UP TO 10 TIMES IN THE DIRECTION OF EXTRUSION, THE MANDREL HAVING A CENTRE LINE AVERAGE SURFACE ROUGHNESS OF BETWEEN 20 AND 90 MICROINCHES, BEING TAPERED OVER AT LEAST THAT PART OF ITS LENGTH WHICH IS IN CONTACT WITH THE CONTRACTING TUBE TO ALLOW FOR THE CONTRACTION OF THE TUBE AS IT COOLS TO THE SOLID STATE, THE DEGREE OF TAPER BEING AT LEAST 0.0008 INCH PER INCH OF SAID TAPERED LENGTH, AND HAVING A DIAMETER, NOT GREATER THAN 1 INCH, WHICH IS SMALLER THAN THAT OF THE EXTRUSION ORIFICE, PASSING THE TUBE OVER AND IN CONTACT WITH A RESILIENT SEALING MEMBER IN THE FORM OF A FLAT CIRCULAR DISC POSITIONED DOWNSTREAM OF THE TAPERED PART OF THE MANDREL ON A PLUG SEALING THE BOTTOM OF THE MANDREL, THE POSITION OF THE SEALING MEMBER WITHIN THE TUBE BEING SUCH THAT THE TUBE IS SUFFICIENTLY SOLIDIFIED NOT TO STICK WHEN IT CONTACTS THE SEALING MEMBER, COOLING THE EXTERNAL SURFACE OF THE TUBE WITH A COOLING LIQUID, SAID COOLING LIQUID CONTACTING THE TUBE AT A DISTANCE AT LEAST EQUAL TO THE MANDREL DIAMETER BEYOND THE POINT AT WHICH THE TUBE FIRST CONTACTS THE MANDREL, HEATING THE COOLED TUBE TO ITS STRETCHING TEMPERATURE, INTRODUCING GAS