Filter or tool-changing device for screw extruders

In a screw extrusion device utilised for extruding plastics materials having an inlet end and an outlet end, a housing securedly mounted on said outlet end, said housing defining an axial flow path for said plastics material, guide means securedly mounted in said housing and extending perpendicularly to said flow path, filter or tool-carrier means displaceably guided on said guide means, an annular pressure element bearing on said filter or tool carrier so as to secure and seal said filter or tool carrier in an operative position, hydraulic pressurizing means acting on said annular pressure element, said hydraulic pressurizing means including an annular pressure chamber, the improvement wherein said annular pressure chamber has inner and outer wall members, said wall members each having first and second ends, said first end of each said wall member being mounted on said annular pressure element and said second end of each said wall member being mounted on said housing, said inner and outer wall member each being a metal bellows, said bellows being disposed concentrically with one another.