Abstract: Various embodiments of the present disclosure provide a void filler and a hollow door employing the void filler as a door core. Generally, the void filler expands from a compact collapsed state into an expanded state in which the void filler has an open-cell structure including three groups, such as columns, of six sided cells, such as generally hexagonal cells. The cells of the outer columns are substantially the same size, while the cells of the middle column are wider than the cells of the outer columns.