Method of manufacturing valve seating rings

Claim 1

In the manufacture in plastics material of a ball valve seating ring having a countersink formation formed around the bore of the ring, the method comprising: molding the seating ring to substantially the desired shape and dimensions in a mold, removing the molded seating ring from the mold, inserting a frusto-conical thrust ring into the countersink formation in the molded seating ring, positioning the molded seating ring in a jig which presents to such seating ring spherical and cylindrical surfaces corresponding, respectively, to the ball surface and the bore wall surface with which the seating ring is to cooperate in use, applying pressure to said thrust ring to displace it axially of the seating ring towards the interior of said jig whereby to compress said seating ring axially to cause the material of the seating ring to undergo cold-flow into intimate contact with said spherical and cylindrical jig surfaces, machining an exposed end face of said thrust ring while the assembly of thrust ring and seating ring are in the jig and after said axial compression, releasing the pressure on said thrust ring and removing the molding shaped and machined seating ring from the jig.