Abstract: A sandwich panel has a core formed of a plastic sheet having columns of equally spaced points displaced from one side, and other columns of similarly spaced points displaced from the other side, the columns on the two sides being interleaved and the points in adjacent columns on the same side being staggered, with the points in the other-side columns being disposed in rows with the oncoming one side column. The displaced points on a side form somehat cone-like structures which intersect the cone-like structures formed by displaced points on the other side to form structures constituting tetrahedrons and octahedrons when assembled and cemented between the layers of a panel. An apparatus and method for forming the core from a heated plastic sheet, involve passing the sheet between two rollers having projections which displace the points to each side. Another apparatus and method involve forming core shapes over stationary projections. Complex and compound structures may be created.