AbstractInstability in melt flow initiates within capillaries for some samples of the two types of polymers linear polyethylene and copolymers of tetrafluoroethylene and hexafluoropropylene. This behavior contrasts with a previously described instability which initiated at capillary inlets. The instability initiating within capillaries results, initially, in fine‐scale surface roughness and, ultimately, in slip or plug flow. Instabilities initiating at both sites appear to result from elastic failure or melt fracture at critical elastic strains of about 5 units. Data supporting these conclusions were obtained from capillary flow and flow birefringence, and rotational viscometr
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