longitudinal bores 20 and 20 39 in the housing headpiece 7 as well as with one another at their central shafts 22 such that the screws effect a scraping action on the walls of the housing 7 and on one another for this purpose the housing headpiece 7 is formed of an upper part 18 and a lower oval part 19 which partially surrounds the degassification screws 17 17 39 the arrangement is such that the periphery 46 of the threads of the degassification screws 17 17 39 extends with slight clearance at the lowest ends in the screw 3 of the screw extruder 1 and elsewhere with slight clearance in the longitudinal bores 20 20 39 of the oval part 19 in order to convey the gas or vapor streams to the outlet 8 the upper part 18 of the housing headpiece 7 includes a conically tapered wall 29 joining the lower part of the housing 7 to the outlet 8 in particular the lower end of housing headpiece 7 is rectangular in outline and wall 29 connects the rectangular outline of the housing to the outlet 8 which is cylindrical in order to effect heating of the housing headpiece 7 the housing headpiece 7 has a heating chamber 21 in oval part 19 for flow of a heating fluid the housing headpiece 7 is provided with an open mounting plate 24 for attachment.