Process and relative machine for moulding a thermoplastic material for producing a finished article having a shape, wherein the process comprises: - providing a mould (99) comprising a first (1) and a second half-mould (2) each having a respective conformation surface (3, 4) which define, with closed mould, a cavity (5) having, in a conformation configuration, the shape; - making a semi-finished product (70) made of the thermoplastic material; - closing the mould (99) with the semi-finished product (70) interposed between the conformation surfaces (3, 4); - admitting a heating fluid into the cavity (5) for heating the semi-finished product (70) permeable to the heating fluid; - arranging the cavity (5) in a compacting configuration in which the cavity (5) coincides with a sub-portion of the cavity (5) in the conformation configuration; - with the cavity (5) in the compacting configuration and the heating fluid into the cavity (5), compressing the semi- finished product (70) between the conformation surfaces (3, 4); - subsequently to compressing the semi-finished product (70), arranging the cavity (5) in the conformation configuration while keeping the heating fluid into the cavity (5), wherein the semi-finished product (70) elastically expands for completely occupying the cavity and assuming the shape; - subsequently, evacuating the heating fluid from the cavity (5) for cooling the semi-finished product (70) for obtaining the finished article with the shape; - opening the mould (99) and extracting the finished article from the mould (99).