Abstract: An annular groove is formed in the periphery of thermoplastic pipe near an end thereof by combining the forces of a fluid under pressure and a rigid expandable core mold. A pipe is heated, softened and placed over a core mold having an extensible portion. The extensible portion is in the retracted state initially and presents no obstacle to the placement of the pipe on the core mold. A space is provided internally of the pipe and is sealed by parts of the core mold whereby the application of a fluid pressure pushes outwardly on the inner periphery of the pipe. At the same time or subsequently the extensible portion of the core mold is extended to provide additional pressure to and to shape the latter internal periphery of the pipe.