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 .