In this research, highly open-cell low-density polyethylene sheet foams are achieved with an annular die by applying various strategies for cell opening, i.e. (i) creation of a structural nonhomogeneity consisting of hard and soft regions with partial crosslinking, (ii) blending of a hard second-phase material (i.e., polystyrene phase) into the low-density polyethylene matrix, (iii) plasticization of the soft region with a secondary blowing agent, (iv) decrease of the cell wall thickness by increasing the cell density, and (v) decrease of the cell wall thickness by increasing the expansion ratio while cell walls are soft. Although the higher surface-to-volume ratio of the sheet foams compared with filament foams made it challenging to prevent gas loss, highly open-cell (up to 99%) and microcellular (up to 3.5 x 10(10) cells/cm(3)) foam sheets were successfully manufactured with high-pressure annular dies using the cell-opening strategies. (c) 2006 Wiley Periodicals, Inc.