In a machine for moulding of composite plastic material, in particular of foamed polystyrene coupled with rigid polystyrene sheet, the rigid material (2) is preformed at a performing station, with the preformed portion being simultaneously separated from the starting material, and the preformed shaped portion is transferred from the preforming station to a moulding station, in which the foamed material is moulded by sintering and pulsed steam action on the preformed rigid material. The resulting piece is brought back to the preforming station (1) and a further portion of rigid material (2) is preformed on the foamed material, and this portion is then thermoset inside the moulding mould (3) on the foamed material, thus generating an article with two opposite surfaces made of a rigid material, between which the foamed material is sandwiched.