Die lines are annoying surface striations that can destroy extrudate aesthetics and compromise product properties. The available literature on die lines has been previously reviewed to explore how die lines arise and how to minimize them. Many factors have been found to influence die line formation. Some techniques are effective in suppressing die lines, mainly discovered through manufacturing experience. Here, we first briefly introduce how to measure and characterize die lines (die line metrology). Film blowing experiments are then conducted to study how various die defects, material properties and operating conditions affect die lines. The empirical relations between die lines and die defects, material properties and operating conditions are explored using dimensionless groups. We also investigate the fluid mechanics in die line formation. The free surface flow of polymer melt leaving the die lips is simulated. The qualitative relations between die defects and die lines are explored. The results are compared with film blowing experiments. Finally, new ways to suppress die lines and prospects for future studies are proposed. (C) 2004 Society of Plastics Engineers.