ABSTRACT
Flow instabilities that occur in melt processing arise from a combination of polymer viscoelasticity and the large stresses that occur in large, rapid deformations. This is in contrast to flows of Newtonian fluids, where inertia and surface tension are usually the driving forces for flow instabilities. Both the elasticity and the high stresses that occur in the flow of molten polymers arise from their high molecular weight (i.e., from the enormous length of their molecules). The high stresses are associated with the high viscosity of molten commercial thermoplastics and elastomers. Typical values range from 103 to 106 Pa s, whereas the viscosity of water is about 10!3 Pa sec. An easily observed manifestation of melt elasticity is the large swell in cross section that occurs when a melt exits a die.
