Patent GB-1325767-A (GB)
Title: Formation of thermoplastic film
IPC: B29C48/90
Filed: 1970-09-02
Abstract: 1325767 Tubular films ESSO RESEARCH & ENG CO 2 Sept 1970 [24 Sept 1969] 41996/70 Heading B5B A process for the production of tubular films comprises extruding a molten thermoplastic resin downwardly from an annular die, passing the film around a vertically extending substantially cylindrical mandrel having an outside diameter approximately equal to the inside diameter of the film and arranged in a surrounding annular cooling cell, the film passing through the cooling cell wherein a first stream of cooling liquid (e.g. water) is passed in a turbulent zone in turbulent countercurrent flow in contact with the lower outside surface of the tubular film and a second stream of cooling liquid is passed in a quiescent zone in cocurrent laminar flow in contact with the upper portion of the outside surface of the film, the first and second streams of cooling liquid being withdrawn from the cell in a mixing zone adjacent the surface of the tubular film at a medial point in the cell. The film may be partially cooled and expanded to a diameter larger than that of the mandrel prior to contact therewith by a pre-cooling gas (e.g. air) directed upwardly from inside the mandrel, the gas also serving to prevent volatile products from condensing on the surface of the mandrel. The partially cooled tubular film is held in contact with the mandrel surface by pressure exerted on its exterior surface by the streams circulating cooling liquid in the cell. Preferably the tubular film is of polyethylene, when the temperature of cooling liquid in the quiescent zone is higher than that in the turbulent zone, or polypropylene when the quiescent zone liquid is cooler than the turbulent zone liquid. Apparatus for use in the process comprises an extruder for tubular films (not shown), a cooling cell 16 adapted to circumscribe an upstanding cylindrical mandrel 17 over which the film 15 is moved parallel to the main axis of the cell, and an outlet 36 in the upper part of the mandrel for directing cooling gas against the inside surface 34 of the film. The cooling cell 16 comprises an outer wall 40 spaced from the mandrel 17; a first upstanding inner wall 41 within the outer wall surrounding and narrowly spaced from the lower part of the mandrel and forming therewith a first annular passage 42 which defines the turbulent zone in the cell wherein cooling liquid moves countercurrently over the film surface 35; a second upstanding inner wall 75 surrounding and widely spaced from the upper portion of the mandrel and forming therewith a second annulus 76 which defines the quiescent zone in the cell for cocurrent laminar flow of cooling liquid over the film surface; a mixing zone 60 defined between the first and second inner walls 41 and 75; an opening 47 for admitting cooling liquid to the lower end of the turbulent zone 42 via deaerating plenum chamber 46, opening 49, packed bed 50, chamber 52 and passageway 52; an opening 57 for admitting cooling liquid to the upper end of the quiescent zone 76 via chamber 54 and passageway 73; and outlet 78 for withdrawing cooling liquid from mixing zone 60 via passageway 77. Resilient seal 63 urges the film 15 against the mandrel 17 by fluid pressure through conduit 70 on pressure ring 69 and also serves to retain liquid in the cooling cell. In operation the liquid level in the cell preferably touches the cover-plate 55 and, there being little disturbance in the liquid at the top of the cell, cover 55 and mandrel 17 generally define a static zone 80 through which the film passes before reaching the quiescent and turbulent zones. The mandrel 17 may be internally cooled with water maintained at a temperature of 2 to 75 C.