The processability of polymers by extrusion is closely related to the interface between the polymer melt and the die wall. The wall surface energy affects the flow of polymer melts in extrusion die. For the purpose of improving the processability in such a process, the effects of boron nitride (BN) powders on the rheological properties and the extrusion processability of metallocene-catalyzed low-density polyethylene (m-LDPE) have been investigated in this study. It is known that BN powders act as a solid lubricant, and lower the surface energy. Two kinds of BN particle, differing in crystal and agglomerate size, have been used and compared. The initial agglomeration of particles had little effect on extrusion. However, crystal size and size distributions significantly influenced the processability. Small crystal size and uniform size distribution were found to be more effective. Also, this improvement of processability was due to the increased slip velocity, not due to rheological property changes. The effect of a hot-pressed BN die on the instability of capillary flow was also studied. A hot-pressed BN die was quite effective in delaying surface and sharkskin defects and postponing gross melt fracture compared to a tungsten carbide die. A synergistic effect of processability improvement could be obtained when both BN powder and a hot-pressed BN die were used together. (C) 2003 Wiley Periodicals, Inc.