A two-sided molded plastic core structure made of polymeric materials comprising a panel having first and second parallel sides, each side having formed therein a regularly spaced array of cell-like receptacles with side walls and floors arranged in rows and columns on both sides of a single layer of plastic, the receptacles on one side interfitting with inverted receptacles on the opposite side in a syntactic fashion, the receptacles are three dimensional and symmetrical in the sense that they look the same, although offset, when viewed from obverse and reverse sides, the receptacles in one side being inverted relative to the receptacles on the other side, said inverted receptacles being nested between the non-inverted receptacles, characterized in that the receptacles are formed with full depth, inwardly sloping major walls (12, 102, 104, 106) that are common only with receptacles on the opposite side of the structure, whereas alternatingly interspersed between the uppermost portions of the full depth major walls (12, 102, 104, 106) are partial depth minor walls (16, 110) forming ribs between adjacent receptacles, the minor walls (16, 110) being substantially vertical, i.e. the minor walls (16, 110) are within five degrees or so of being vertical when the panel is flat and lying on a horizontal surface, and are common only to neighboring minor walls (16, 110) on the same side of the core structure, the major walls (12, 102, 104, 106) extend down to floors (14', 108), whereas the minor walls (16, 110) do not, wherein the minor walls (16, 110) are essentially triangular with an apex, the apex of a minor wall (16, 110) on the obverse side being essentially aligned with the apex of an inverted minor wall (16, 110) of the structure on the reverse side so as to create a continuous beam structure extending from one side of the core structure to the other. the receptacles on one side interfitting with inverted receptacles on the opposite side in a syntactic fashion, the receptacles are three dimensional and symmetrical in the sense that they look the same, although offset, when viewed from obverse and reverse sides, the receptacles in one side being inverted relative to the receptacles on the other side, said inverted receptacles being nested between the non-inverted receptacles, characterized in that the receptacles are formed with full depth, inwardly sloping major walls (12, 102, 104, 106) that are common only with receptacles on the opposite side of the structure, whereas alternatingly interspersed between the uppermost portions of the full depth major walls (12, 102, 104, 106) are partial depth minor walls (16, 110) forming ribs between adjacent receptacles, the minor walls (16, 110) being substantially vertical, i.e. the minor walls (16, 110) are within five degrees or so of being vertical when the panel is flat and lying on a horizontal surface, and are common only to neighboring minor walls (16, 110) on the same side of the core structure, the major walls (12, 102, 104, 106) extend down to floors (14', 108), whereas the minor walls (16, 110) do not, wherein the minor walls (16, 110) are essentially triangular with an apex, the apex of a minor wall (16, 110) on the obverse side being essentially aligned with the apex of an inverted minor wall (16, 110) of the structure on the reverse side so as to create a continuous beam structure extending from one side of the core structure to the other. the receptacles are formed with full depth, inwardly sloping major walls (12, 102, 104, 106) that are common only with receptacles on the opposite side of the structure, whereas alternatingly interspersed between the uppermost portions of the full depth major walls (12, 102, 104, 106) are partial depth minor walls (16, 110) forming ribs between adjacent receptacles, the minor walls (16, 110) being substantially vertical, i.e. the minor walls (16, 110) are within five degrees or so of being vertical when the panel is flat and lying on a horizontal surface, and are common only to neighboring minor walls (16, 110) on the same side of the core structure, the major walls (12, 102, 104, 106) extend down to floors (14', 108), whereas the minor walls (16, 110) do not, wherein the minor walls (16, 110) are essentially triangular with an apex, the apex of a minor wall (16, 110) on the obverse side being essentially aligned with the apex of an inverted minor wall (16, 110) of the structure on the reverse side so as to create a continuous beam structure extending from one side of the core structure to the other.