A temperable screw casing for a twin screw extruder for processing plastics comprises a supporting member and wear resistant liner disposed therein. The liner has a spectacle shaped bore accommodating both screws of the extruder. The supporting member, which engages around the liner, takes over the reinforcing function of the screw casing. Low technical manufacturing cost and satisfactory tempering of the screw casing is achieved by providing the liner with a circular external cross section, surrounded by a supporting member which has a circular internal recess. Axial tempering holes are provided in wedge shaped projections which extend into the spectacle shaped bore of the liner. Axial tempering holes are disposed opposite one another in the supporting member, at the sides of the spectacle shaped bore of the liner. These result in individual tempering systems, by which the liner and supporting member can be tempered independently.It is known to use twin screw extruders for processing plastics materials where the internal processing chambers are formed by wear resistant liners. The liners have internal layers which are wear resistant to lengthen the life of the extruding cylinder. To enable the liners to be replaced and to absorb the forces occuring in operation, the liners are surrounded by a supporting member.
When tempering casings of twin screw extruders of this type, that is to say controlling the temperature thereof, difficulty is experienced in obtaining a good even tempering action over the periphery of the spectacle shaped combined bore. The greatest quantity of heat per unit appears in wedge shaped projections between the two longitudinal bores of the liner, while lateral portions of the spectacle bore show less localisational heat.
In a known tempering means for twin screw extruders of this type, spiral external grooves are formed in the periphery of the liner. A spiral tempering passage is thus formed upon insertion of the liner into the supporting member. However, forming a good seal between the liner and the supporting member has been found to be a problem. Since the tempering medium has to be passed through the spiral tempering passages under pressure, and since a satisfactory sealing action cannot be obtained because of the need to be able to replace the liner, this known solution has rarely been used, German Published Application 20 61 700.
According to the invention there is provided a screw casing for a twin screw extruder for processing plastics, comprising a wear resistant liner having two parallel longitudinal bores therein with the bores overlapping and forming a combined spectacle shaped bore having two generally wedge shaped projections, projecting thereinto, said bore receiving twin extruder screws, and further comprising a supporting member which surrounds said liner and has a reinforcing function, wherein said liner has a circular external cross section, said supporting member is cylindrical and has a circular section bore in which said liner is received, axially extending tempering passages are formed in said generally wedge shaped projections of said liner and further axially extending tempering passages are provided in said supporting member at positions opposite one another, at the sides of said combined spectacle shaped bore.
