A mixing device comprises a cylindrical body (14) to be terminally engaged with an extrusion screw (2) and carrying at least one primary mixing thread (15) and at least one secondary mixing thread (16) having a crest diameter (d₃) smaller than the crest diameter (d₁) of the primary mixing thread (15). The primary mixing thread (15) has a helical extension in the same direction as that of the secondary mixing thread (16), to delimit at least one primary flow channel (17) and at least one secondary flow channel (18) that are substantially parallel. The primary mixing thread (15) further has, on its crest (19), communication grooves (20) between the primary (17) and secondary (18) flow channels.