Method and device for forming an internal annular groove in a plastic tube part

A method for forming an internal annular groove in a plastic tube part by sliding the part onto a core and surrounding same with a mold having an annular recess, heating the tube part and forming the groove by applying an expandable and retractable segmented rigid groove forming a ring in a recess in the interior of the tube part which ring is removed by a retracting action after forming the final groove and cooling down the plastic tube part, subjecting first the plastic tube part to the action of an internal medium overpressure in order to form a groove with a bottom having an inner circumference which is greater than the periphery of the completely expanded groove forming ring, performing an upsetting action simultaneously with the action of said internal medium overpressure and after having formed the groove by means of the internal medium overpressure moving the groove forming ring to its completely expanded position, whereafter the inner side of the groove which was formed by the internal medium overpressure is shrunk onto the completely expanded groove forming ring and the recess for expanding the groove forming ring is closed by a sleeve which is pushed away during sliding of the plastic tube part onto the core.