Method of producing undercut tubular synthetic plastic articles

A method of making a tubular pipe fitting of the type having a longitudinally extending axis and having a predetermined wall thickness throughout and wherein two outer circumferentially extending annular portions having first inner diameters flanking an intermediate circumferentially extending annular portion having a larger second inner diameter defining a circumferentially extending annular groove for receiving a gasket, packing or the like, the difference between the radii of said outer annular portions and the radius of said intermediate annular portion being within a range of more than to at least equal to said thickness, comprising the steps of introducing plasticized synthetic plastic material into a mold whose cavity has a configuration matching that of the fitting, the mold being shiftable between an open position to facilitate removal of the fitting and a closed position to form the cavity and facilitate formation of the fitting, the mold cavity being defined by a core having a first core part for cooperating in forming one of the outer annular portions and a second core part for cooperating in forming the other of the annular portions together with the intermediate annular portion, the core parts being axially shiftable relative to one another between a first position at which the core parts cooperate in forming the mold cavity and second position at which the second core part is adapted to be stripped from the intermediate annular portion of the fitting, the mold cavity also being defined by a first outer mold part radially shiftable between a first position at which it cooperates in forming part of the cavity for one of the outer annular portions and part of the second intermediate annular portion and second radial position away from the fitting to permit stripping of the core, the mold cavity also being defined by a second outer mold part shiftable axially between a first position at which it cooperates in forming part of the cavity for the other of the outer annular portions and the remainder of the intermediate annular portion and a second axial position away from the fitting to permit stripping of the core, the mold cavity surrounds the core including a larger portion with an outer diameter corresponding to said second inner diameter and two smaller portions flanking said larger portion and having outer diameters corresponding to said first diameters whereby the introduced material is converted into said fitting;

• exposing the exterior of the fitting by shifting the first and second outer mold parts to their respective second position, at least in the region of said intermediate portion and one of said outer portions;
• maintaining the latter one outer portion of the exposed fitting in a state in which it can undergo elastic deformation;
• and moving the fitting and the second core part relative to each other in a direction to force the larger portion of the core through said latter one outer portion of the fitting whereby this one outer portion expands during the passage of said larger portion therethrough and thereupon contracts so that its inner diameter is again less than the inner diameter of said intermediate portion.