passing the seal and coming into contact with the cooling fluid this controlled atmosphere avoids air leakage past the seal and consequent imperfections in the thermoplastic material tube application_number US 70580976 A description referring now to the blown tube apparatus 10 of fig1 polypropylene resin in the form of resin pellets 11 are fed into a hopper 12 and then into an extruder 13 where they are heated to form a very soft or molten mass of polypropylene this mass is extruded from the extruder 13 through a die 14 and comes into contact with a cooling mandrel 15 where it begins to crystallize in the form of a stalk 16 from the cooling mandrel 15 the stalk 16 passes through a pair of nip rolls 17 which compress the stalk in sealing relationship to an air tube 18 which passes therebetween in a groove in the rolls 17 after passing the rolls 17 the stalk 16 is reheated to its softening temperature by a suitable heating means 19 for example radiant heaters and then insufflated by introducing air under pressure through tube 18 insufflation provides a controlled bubble or large tube 20 which stretches the polypropylene stalk in both the horizontal and vertical direction by a factor of 6 to provide biaxially oriented polypropylene film the bubble 20 is then collapsed between a further pair of nip rolls not shown and hauled off to a slitter where the bubble is slit into one or more widths which are then wound on a takeup winding roll typical structures in processes for producing films from a blown tube process are found in u s.