| 摘要 |
A chimney member that is used with an air ring employed to cool a blown extruded tube of molten plastic material, and also to stabilise the moving tube, surrounds the tube as it emerges from the die to keep the cooling air in contact with the tube wall. Too high air velocity results in instability of the bubble, resulting in non-uniform film, or even disruption of the bubble; the air volume and velocity must therefore be restricted. The side wall of the chimney member is of inverted frusto-conical shape diverging outwards in the direction of bubble movement and spaced a minimum distance from the tube outer wall. In one embodiment it is provided over substantially its entire area, preferably except for a narrow portion at its downstream edge, with a large plurality of uniformly distributed longitudinally and circumferentially spaced apertures, which permit the air in the passage between the chimney and tube to escape and reduce the venturi force it produces to a safe value. In another embodiment the apertures take the form of longitudinally-spaced annular circumferential slots formed between annular wall portions and in a further embodiment they take the form of circumferentially spaced longitudinally extending slots. The air velocity and volume can now be increased substantially without producing vibration, so that output is increased.
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