The invention relates to a mold for producing three-dimensional items, consisting of at least two pieces that can be attached together and within which pieces is delimited the volume of the item to be produced. According to the invention, owing to the mutually supporting surfaces, said pieces have formations, formed of consecutively attached pieces, delimiting airtight chambers. At least one of the pieces used to form part of each airtight chamber has a pneumatic valve via which said chambers are connected to a vacuum source, by means of which a sufficient pressure differential is generated in the chambers to fix together the pieces delimiting the chambers. The walls of the pieces are non-deformable as a result of the pressure differential created within the chambers.