a six - sided or parallelepiped carton which is formed by folding a die - cut blank of a generally rigid , unitary blank of a polymeric material in which a major portion of the thickness of the blank comprises an expanded polymeric material , at least some of the corners of such carton being self - sealed by webbed areas or gussets which comprise portions of such blank but which are of reduced thickness relative to the major portions of the remainder thereof , and a die - cut blank which is adapted to be formed into such a carton .