An annular die is the circular die on a blown film line that releases the melt in the form of a thin tube. Inside the die, the melt is distributed around the circumference through channels and emerges upwards through a narrow ring-shaped gap. Air blown into the tube inflates it like a balloon.
For the film thickness to be even, the die must distribute the melt evenly around its circumference. Small differences in the die gap can show up in the film as thick and thin bands. On some lines, the die or the haul-off unit rotates or oscillates to spread these differences across the film. In co-extrusion dies, more than one melt is combined in layers inside the die.