Stretching flow instabilities at the exits of extrusion dies

As liquid leaves an extrusion die, the surface layers are rapidly stretched. Stretching flows may become unstable in two ways: by breaking, or in a ductile manner producing an uneven “necked” sample which, in continuous extrusion and drawing, is sometimes called “draw resonance”. There is a quantitative correlation between the extrusion defect known as “sharkskin” and the cohesive failure of polymer melts. By extruding under closely defined conditions, it is possible to introduce a transitory “structure” into the surface layer of the extrudate greatly enhancing its cohesive strength and eliminating this defect. A similar quantitative correlation is established between the uneven coating thickness sometimes obtained during coextrusion of a high viscosity melt on the surface of a low viscosity melt and the tensile drawing instability known as “draw resonance”. Simple criteria are established to avoid this problem in practical flow engineering.