GB1045899A - A process and apparatus for cooling tubular thermoplastic films - Google Patents

Abstract

1,045,899. Films. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ. N.V. Oct.13, 1964, No. 41730/64. HeadingB5B. In a process for cooling an extruded tubular film of an organic thermoplastic polymeric material by blowing cooling air on to the outer surface of the extruded tube whilst the outer diameter thereof is increased by inflating to form the tubular film, the whole of the cooling air is directed to flow through a single outlet zone to the atmosphere, said zone surrounding the extruded tube in a region where the ratio of the average outer diameter of the tube to the outer diameter of the annular opening of extrusion die (d 2 d 1 ), and to the final outer diameter of the tubular film (d 2 : d 3 ) is at least 1.5 and less than 1 respectively, in such a manner that the dynamic pressure of the cooling air in this zone is increased to such an extent that the static pressure is decreased to a subatmospheric value. The ratio d 2 : d 1 is desirably at least 2. Preferably the cooling air is supplied from an air ring contiguous to the extrusion die and passed through a tapering annular zone between the outer surface of the tube and the inner wall of the air ring. The inner wall of the air ring may be cylindrical, the tapering annular zone being formed between this wall and the outwardly diverging wall of the film, but preferably diverges in the take-off direction of the tube (e.g. a conical shape) and has at least one annular slit, preferably two or three, circumferentially arranged around the tube, through which the cooling air issues on the surface of the tube. A ring having a number of discontinuous annular slits extending over a certain angular range of the inner wall of the air ring and which overlap one another may also be used. The cross-sectional area of the slits may be adjustable, suitably during operation, and the various annular slits may be shaped in such a manner as to permit the cooling air to be discharged therefrom at different angles, these angles being relative to the central axis of the air ring. Thus the discharge angle of the annular slit which is situated at the greatest distance from the zones in which the static pressure is subatmospheric is from 70 to 90 degrees, relative to the central axis of the air ring. In Fig. 1, a tube 1 is extruded through the annular opening 2 of the extrusion die 3 and is inflated by air supplied through blow line 5 to form a tubular film. Air ring 10 has a frustum-shaped inner wall 11 and is supplied with compressed air, which blows through the openings 12 into the annular space 14 situated between the air ring and the curving outer surface of the tube, which space is tapered in the direction 15 in which the cooling air is discharged to the open atmosphere. Hence the rate at which the air flows upwardly through the annular space 14 is increased, the highest rate occurring in the passage Z which forms a Venturi zone. The tapering effect of the space 14 and the amounts and rates of the cooling air discharged from the openings in the inner wall of the air ring are balanced so as to give such increased rates and hence increased dynamic pressures providing for a sufficient decrease of the static pressure of the cooling air flowing through this zone as indicated in Fig. 1 by arrow 15 as to lower the latter pressure to a subatmospheric value. The subatmospheric pressure exerts a noticeable pulling effect resulting in a stabilization of the position of the film. Also, as the wall is pulled evenly outwards, the surface of the tube shows no tendency whatsoever to wrinkle or flutter. In Fig. 2, the air ring has discharge openings shaped so that the angle at which the cooling air issues from the lower openings is less acute to the central axis of the tube than the angle of the higher openings. Preferably the discharge angle of the lowest slit 16 is 70-90 degrees and that of the highest slit 17 is 20-0 degrees with regard to the central axis of the tube. Such inclined slits provide better guidance of the cooling air. Alternatively, the same effect may be obtained using horizontal slits of which the lower border walls are extended with air deflecting plates. Figs. 3 and 4 (not reproduced) illustrate angled slits the cross-sectional area of which may be adjusted during the cooling operation. Rings 18, 19 and 20 in Fig. 3 are connected to stays 21 by screw-threads, and may be turned upwardly or downwardly about their vertical axis to adjust slit width. In Fig. 3, relatively thin stays securing the position of the separate rings are used to prevent blind spots in the stream of cooling air. In Fig. 4, relatively thick stays are used but these are provided with openings 22. A further feature of Fig. 4 is that the separate rings and slits are not positioned within the periphery of the air ring. Using the separate rings 23, 24 and 25 the top end of ring 25 may be placed above the upper circular wall of the air ring. In general, by modifying ring construction and the blow-up ratio of the films the position of the Venturi zone relative to the inner wall of the ring may be changed. The discharge of cooling air from the openings should be radially homogeneous. This is secured by using deflecting devices e.g. 27, 28 in Fig. 3. Specified materials include polyethylene, polypropylene, polystyrene and polyvinyl chloride.