Studies on blown film extrusion. III. Bubble instability

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Abstract
An experimental study has been carried out to investigate flow instabilities in blown film extrusion. Two types of flow instabilities were observed, depending on whether a bubble was under uniaxial or biaxial stretching. Under biaxial stretching, the phenomenon of a surface wave‐type instability was observed, yielding wavy bubble shapes which very much resembled water waves at the free surface. Under uniaxial stretching, another type of instability, frequently referred to as draw resonance, was observed. It was also observed that, once draw resonance occurs, the amplitude and frequency of bubble diameter pulsing increased with stretch ratio. Quantitative information was obtained from a series of motion pictures taken of bubble diameter in both types of flow instability. It was observed further that an increase in extrusion melt temperature enhanced the severity of bubble instability.

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