Apparatus for producing double-walled tubes of plastic material

Apparatus for producing a double-walled tube of plastic material having a corrugated outer wall and an inner wall which at least in longitudinal direction of the tube has an inner surface extending parallel to the axis of the tube, said apparatus comprising mold means comprising a plurality of mold sections having inner corrugated surfaces and being movable along endless paths and abutting along portions of said paths against each other to form a tubular mold;

• an extrusion head extending into said tubular mold coaxially therewith and having a first annular extrusion nozzle having an outlet end and a second annular extrusion nozzle coaxial with said first annular extrusion nozzle and having an outlet end downstream of said outlet end of said first extrusion nozzle;
• passage means for gas under pressure in said extrusion head and having a discharge end between the outlet end of said first and that of said second annular extrusion nozzle to maintain between said outlet ends an overpressure for pressing tubular plastic material emanating from the outlet end of said first nozzle against the inner corrugated surface of said tubular mold, each of said annular extrusion nozzles comprising an inner core and an annular outer wall spaced from the coaxial with the respective inner cord, the outer wall of the second extrusion nozzle projecting from the inner core of said first extrusion nozzle and being downstream of the outlet end of said first extrusion nozzle and the discharge end of said passage means enlarged to a sealing piston having a cross-section substantially equal to the inner free cross-section of said outer wall and an axial length which is a multiple of at least twice the distance between successive inner crests of the corrugations of said outer wall to thus substantially prevent passage of gas under pressure beyond the downstream end of said sealing piston;
• pressure equalization passage means in said injection head and having an inlet end in said outer wall of said second nozzle adjacent the outlet end of the latter for permitting discharge of any gas under pressure which has passed to the downstream side of said sealing piston;
• and means downstream of the outlet end of said second extrusion nozzle for pressing said tubular plastic material emanating from the outlet end of said second nozzle onto the inner crests of said corrugated outer wall.