Expandable polystyrene composition, expanded beads and moulded parts

A pre-expansion process making it possible to increase the rate of expansion of beads of a homogeneous composition of an expandable styrene polymer comprising (a) 100 parts by weight of a styrene polymer having a weight-average molecular mass, Mw, of 150,000 to 300,000 and a residual monomer content of less than 2000 ppm, (b) from 2 to 9 parts by weight of at least one blowing agent comprising n-pentane, (c) from 0.1 to less than 1.0 part by weight of a petroleum wax comprising a mixture of C18 to C80 alkanes and (d) from 0 to 1.0 part by weight of a nucleating agent chosen from synthetic Fischer-Tropsch or polyolefin waxes, said homogeneous composition being prepared in part by polymerizing styrene in an aqueous suspension to obtain the styrene polymer in the presence of the petroleum wax, the petroleum wax being introduced into the aqueous suspension at a time when the conversion of the styrene into styrene polymer is from 0 to 35%, whereby the petroleum wax is distributed uniformly throughout the composition, the process comprising the step of mixing and heating the homogeneous composition with steam in one or more passes through a stirred tank.