A method for producing a sealed container with a product to be preserved therein, wherein a tray is folded from a blank, wherein the blank comprises: a polygonal bottom part, side walls (3, 4, 5, 6) connected along folding lines with the sides of said bottom part, at each corner of said bottom part, two triangular corner connecting parts (7, 8) connected along folding lines with each other and two adjacent ones of said side walls (3, 4, 5, 6), and flange parts (9, 9', 10, 11, 11', 12) connected along folding lines with each of said side walls (3, 4, 5, 6) and with at least one of each said two triangular corner connecting parts (7, 8), wherein at least said flange parts (9, 9', 10, 11, 11', 12) have a plastic upper side, wherein said tray is folded by: folding said side walls (3, 4, 5, 6) in one direction relative to said bottom part, and at each corner of said bottom part, folding and fixing said two triangular corner connecting parts (7, 8) onto either the inner side or the outer side of one of the adjacent side walls (3, 4, 5, 6), folding said flange parts (9, 9', 10, 11, 11', 12) outwardly such that a substantially flat closed-circumferential flange (9, 9', 10, 11, 11', 12) is formed at the upper side of the tray, wherein said tray is placed in a die (20, 21) with said product present in said tray, wherein a sheet is placed over the flange (9, 9', 10, 11, 11', 12), and wherein a stamp is pressed against said sheet and said flange (9, 10, 11, 12) while the lower side of the flange abuts a counter press surface of the die (20, 21), whereby said sheet is sealed against said flange (9, 9', 10, 11, 11', 12), characterized in that said stamp is provided with local protrusions near each corner, wherein seen from above each of said protrusions extend across and on both sides of a step which is formed by the edge of a flange part (10, 12) which extends on top of the flange part (9', 11') below it, such that during sealing the pressure on said sheet and said flange parts (9', 10, 11', 12) is higher across and on both sides of said step than in adjacent parts of said flange parts (9', 10, 11', 12). a polygonal bottom part, side walls (3, 4, 5, 6) connected along folding lines with the sides of said bottom part, at each corner of said bottom part, two triangular corner connecting parts (7, 8) connected along folding lines with each other and two adjacent ones of said side walls (3, 4, 5, 6), and flange parts (9, 9', 10, 11, 11', 12) connected along folding lines with each of said side walls (3, 4, 5, 6) and with at least one of each said two triangular corner connecting parts (7, 8), wherein at least said flange parts (9, 9', 10, 11, 11', 12) have a plastic upper side, folding said side walls (3, 4, 5, 6) in one direction relative to said bottom part, and at each corner of said bottom part, folding and fixing said two triangular corner connecting parts (7, 8) onto either the inner side or the outer side of one of the adjacent side walls (3, 4, 5, 6), folding said flange parts (9, 9', 10, 11, 11', 12) outwardly such that a substantially flat closed-circumferential flange (9, 9', 10, 11, 11', 12) is formed at the upper side of the tray, wherein said tray is placed in a die (20, 21) with said product present in said tray, wherein a sheet is placed over the flange (9, 9', 10, 11, 11', 12), and wherein a stamp is pressed against said sheet and said flange (9, 10, 11, 12) while the lower side of the flange abuts a counter press surface of the die (20, 21), whereby said sheet is sealed against said flange (9, 9', 10, 11, 11', 12), characterized in that said stamp is provided with local protrusions near each corner, wherein seen from above each of said protrusions extend across and on both sides of a step which is formed by the edge of a flange part (10, 12) which extends on top of the flange part (9', 11') below it, such that during sealing the pressure on said sheet and said flange parts (9', 10, 11', 12) is higher across and on both sides of said step than in adjacent parts of said flange parts (9', 10, 11', 12).