a device for feeding an extrusion head for plastic material includes a main extruder connected to a main die for extruding a continuous main flow of material , and an auxiliary feed mechanism associated with the main die and selectively controlled to cover the outer exit flow of the main die with a surface layer . the auxiliary feed mechanism continuously conducts two auxiliary flows , a first flow forming the surface layer and the second flow being mixed with the main flow . an invertor valve permits selective exchanging of the two auxiliary flows such that the second flow can cover the main flow and the first flow can be mixed with the main flow .