During the die flow of metallocene polyethylenes, flow instabilities may occur. Namely, wall slip, "sharkskin," and stick-slip (pressure oscillations) and gross fracture may be obtained depending on the volume flow rate and die geometry. It was reported that fluoroelastomers and boron nitride powders with hexagonal crystal structure can be used as suitable processing aids in melt extrusion processes. Fluoroelastomers at low concentrations act as die lubricants and may eliminate flow instabilities such as surface and stick-slip melt fracture. On the other hand, specific boron nitride powders may not only elin-Anate surface and stick-slip melt fracture, but also postpone gross melt fracture to higher volume flow rates. In this paper, a way for quantitative differentiation of the influence of polymer processing additives on rheological behavior is shown. Standard material functions show no clear-cut differences. However, using multi-wave oscillations with higher strain amplitudes make a quantitative assessmentpossible. (C) 2004 Society of Plastics Engineers.