Workhead for forming a pipe bell at an end of a pipe made of thermoplastic material

A workhead operated by pressurized fluid in an automatic pipe bell-forming machine for forming a permanent pipe bell in a plastic pipe end portion, comprising:

• a die having a cavity for receiving a thermally plasticized end portion of a plastic pipe having a throughbore which opens through one end of said pipe which is associated with said end portion, said die having a shaped internal surface and being provided with flow conduits for a pressurized fluid, which conduits exit from said shaped surface;
• a forming buffer, insertable into said end portion of the pipe and arranged centrally to said die;
• said buffer having a shaped lateral surface and being provided with flow pipes for a pressurized fluid, which flow pipes exit from said lateral surface;
• a flange mounted coaxially and slidably with respect to said buffer and die, and having a striking surface which is associable to an edge of the end portion of the pipe;
• sealing elements located on the shaped internal surface of the die and on said buffer, and arranged to interact respectively with an external surface and an internal surface of the end portion of the pipe, enabling said pipe to be sealingly inserted in said workhead;
• said die, said buffer and said flange together defining a periphery of a forming chamber for said end portion of the pipe, said forming chamber being arranged to be subdivided by the pipe into a first and a second pressure chamber for a fluid;
• the first pressure chamber being arranged to be located between the lateral surface of the buffer and the internal surface of the pipe;
• the second chamber being arranged to be located between the external surface of the pipe and the internal surface of the die;
• the first and second pressure chambers being arranged to be fed by said conduits circumferentially, and successively relative to one another so as to dilate and subsequently to contract the end portion of the pipe, and thereby produce a band of ribbing in said end portion of the pipe.