The application relates to openable extruder heads on extruders for thermoplastic and heat-curable compositions, in which according to the main patent the head parts which are pivotable about fixed fulcrums are supported against the extruder head as resiliently flexible intermediate members by means of pressure medium cylinders directly attached to them. According to the invention the pressure medium cylinders are shielded against undesired effects of thermal convection by thermal insulation attached to the head parts. It is thus possible to eliminate the hitherto constant danger of malfunctions resulting from the formation of vapour bubbles in the hydraulic fluid.
