Memory effects in a non-uniform flow: A study of the behaviour of a tubular film of viscoelastic fluid
The process is used for the manufacture of a thin sheet or film of a thermoplastic material (e. g. polyethylene) from molten material supplied under pressure by a screw extruder. The polymer melt is forced through an annular die and the tubular film so formed is thinned both by blowing and by axial drawing. The tube is formed into a closed bubble by flattening it when it is cool enough to avoid blocking (the tendency of the film to stick to itself) and then the flattened film is wound onto take-up rolls. The axial tension is provided by the driven nip rolls which close the bubble at the top (the process is usually run vertically with the die at the bottom). The blowing is caused by maintaining an air pressure slightly above atmospheric inside the bubble, and this causes an increase in the radius of the tube and stretching of the film in a circumferential direction. Fig. 1 illustrates the process schematically.
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- Pearson, J. R. A. and C. J. S. Petrie, Plastics and Polymers 38, 85–94 (1970c).Google Scholar