In extruding thick polymeric objects such as bars, voids may form as a consequence of fast cooling. As is well known, this phenomenon is due to volume contraction of the inner zone which takes place when the external “shell” is already solidified. Recent experimental results on this effect show the influence of various parameters such as polymer melt viscosity, bar diameter, and operating pressure upon a critical extrusion velocity above which voids are detected. A detailed analysis of the phenomena which are thought to be relevant is presented and compared with the experimental results.
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Titomanlio et al. (1985) studied this question.
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