Production of a cylindrical housing for a double screw extruder

1. A process for producing a barrel for a twin-screw extruder with intermeshing screws from a steel block, wherein the barrel has a wear resistant inner surface, the process comprising the steps of:

producing two partially intersecting bores passing completely through the steel block, the two intersecting bores forming a spectacle-shaped bore having narrowed regions where the two intersecting bores intersect; and

hardening an inner surface of the spectacle-shaped bore with essentially constant depth of penetration by passing an inductor comprising an electrical conductor through the spectacle-shaped bore from one end to the other, the electrical conductor being smaller than but having approximately the same shape as the inner contour of the spectacle-shape bore and being configured such that less energy for heating the inner surface of the spectacle-shaped bore is delivered in the vicinity of the narrowed regions of the spectacle-shaped bore by the longitudinal sections on the periphery of the electrical conductor than in regions on the inner contour remote from the narrowed regions.

2. The process of claim 1 , further comprising the step of passing a cooling device through the spectacle-shaped bore at a distance behind the electrical conductor for accelerated cooling of the inductively heated regions.

3. The process of claim 2 , wherein the cooling device uses one of liquid and gaseous coolant.

4. The process of claim 1 , wherein said step of hardening comprises passing the inductor through the spectacle-shaped bore such that the inductor is at a greater distance from the inner surface in the vicinity of the narrowed regions than the remaining regions.

5. The process of claim 1 , wherein the electrical conductor comprises a plurality of segments and said step of hardening the inner surface of the spectacle-shaped bore with essentially constant depth of penetration comprises independently electrically controlling each of said plural segments such that less energy for heating the inner surface of the spectacle-shaped bore is delivered by the longitudinal sections on the periphery of the electrical conductor in the vicinity of the two narrowed regions of the spectacle-shaped bore than in regions on the inner contour remote from the narrowed regions.

6. The process of claim 1 , further comprising the step of covering the inner surface of the spectacle-shaped bore with a hardenable plating material before the last machining operation of the steel block.

7. An apparatus for the inductive hardening of an inner surface of a spectacle-shaped bore in a steel barrel for a twin extruder with essentially constant depth of penetration by passing the apparatus from one end of the spectacle-shaped bore to the other, the spectacle shaped bore being formed from two partially intersecting bores which extend completely through the steel barrel and having narrowed regions where the two intersecting bores intersect, the apparatus comprising an inductor including an electrical conductor having an outer contour that is smaller than but has approximately the same shape as the inner contour of the spectacle-shaped bore and is configured such that less energy for heating the inner surface of the spectacle-shaped bore is delivered by the longitudinal sections on the periphery of the electrical conductor in the vicinity of the narrowed regions of the spectacle-shaped bore than in regions on the inner contour remote from the narrowed regions.

8. The apparatus of claim 7 , wherein the inductor comprises a continuous electrical conductor that is bent to form a figure-eight shaped outer contour, the figure-eight shaped outer contour having upper and lower regions, a middle region, and two reduced radius regions between the upper and lower regions and the middle regions, respectively, the upper and lower regions being bent into a circular shape, the reduced radius regions having a decreased radius, and the middle region being an oppositely curved region joining the reduced radius regions, wherein a coolant is circulatable through said continuous electrical conductor tube.

9. The apparatus of claim 7 , wherein said inductor comprises a plurality of segments that are each independently electrically controllable.

10. The apparatus of claim 9 , wherein said plural segments form a figure-eight shaped outer contour, the figure-eight shaped outer contour having upper and lower segments, middle segments, and reduced radius segments between each end of the upper and lower regions and the middle regions, respectively, the upper and lower regions being bent into a circular shape, the reduced radius regions having a decreased radius, and the middle region being an oppositely curved region between the reduced radius regions.

11. The apparatus of claim 7 , further comprising a cooling device arrangeable a distance behind said inductor for cooling the surface regions of the spectacle-shaped bore that are heated by said inductor.

12. The apparatus of claim 11 , wherein said cooling device comprises a circulating distributor pipe bent into a figure-eight shape similar to the outer contour of said inductor and adapted to the spectacle shaped bore, said distributor pipe having a plurality of nozzles for allowing a coolant to pass therethrough directed radially outward and obliquely opposite to an intended passing direction of said apparatus through the spectacle-shaped bore.

13. The apparatus of claim 8 , wherein said electrical conductor comprises a copper tube.

14. The apparatus of claim 12 , wherein said coolant comprises a coolant gas.