Nucleation results are presented for polyethylene sheet extrusion while using volatile carbon dioxide withany nucleating agent. Since the extrusion pressure must exceed the system pressure, mechanical superheat was calculated from the processing pressure. The experimental results show a different distribution from the conventional nucleation predictions, based on either chemical superheat or mechanical superheat. Also noted was the shear energy dependence. Various nucleation mechanisms are discussed to address the energy role played by shear in foam extrusion.
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