The invention relates to an injection mould for manufacturing fittings from thermoplastics, and which fittings are provided on the inside with at least one annular groove.  These injection moulds include a separable outer mould and a mould core which can be pushed into the outer mould.  A complementary mould bead for defining the cavity forming the annular groove is configured on the mould core.  The injection mould is formed in the area of the annular groove of the fitting by a plurality of segments that can be assembled together, and the outside of the area of the mould cavity which will form the portion of the fitting surrounding the annular groove is partly formed by the separable outer mould and partly by a mould ring connected to the mould core. 
An apparatus of this type is described in AT-B 387 750.  In this case, the mould ring, seen in the axial direction of the fitting, forms a seam with the separable outer mould that terminates approximately in the center of the area of the cavity surrounding the annular groove of the fitting.  When the outer mould closes, it moves along the end face of the mould ring arranged normal to the longitudinal axis of the mould ring.  If there is the slightest deviation from the ideal state, this results in either the mould being damaged or a gap being present between the mould ring and the outer mould, into which plastics material may penetrate.  In this way, an unsightly web is produced along the outside of the fitting surrounding the annular groove, which is to be avoided. 
DE 94 14 523 U describes another type of injection mould apparatus.  An advantage of the sustained forcing of the mould ring, however is that the mould ring fixed to the core, shown in the embodiment, is no longer needed for protecting the ring segments.  This is correct inasmuch as the sustained forcing in the injection mould of reference DE 94 14 523 U is more functionally reliable than the hydraulic actuation of the segments of the injection mould of reference AT-B 387 750.  With omission of the mould ring, however, a segment under sustained forcing can damage the expensive outer mould, if that segment is distorted for any reason. 
The object of the present invention is to avoid the disadvantages of the prior art described above.  This object is obtained because the separating surface of the mould ring which abuts the outer mould to form a seam between the mould ring and the outer mould terminates at the cavity for forming the fitting at an angle of less than 45xc2x0 with respect to the axis of the mould core. 
In addition, by arranging the separating surface on the mould ring (which forms the seam between the mould ring and the outer mould) parallel to the axis of the mould core rather than normal to it, the problems with not providing an exact fit between the mould ring and the outer mould cease to apply.  If the separating surfaces of the mould ring and outer mould were to meet on the outside surface of the area of the cavity for forming the portion of the fitting surrounding the annular groove, which surface runs parallel to the axis, as was usual according to the prior art, the feature of the invention might cause the mould ring to have a sharp front edge, which would be susceptible to damage.  It is therefore preferably provided that the front mould surface of the mould ring adjoining the separating surface, and which form a portion of the outer surface of the cavity for forming the fitting, is inclined at an angle of more than 30xc2x0, and preferably at an angle of more than 45xc2x0, with respect to the longitudinal axis of the mould core.  In practice, this means that the separating surface does not meet the part of the outer surface of the cavity for forming the fitting which runs parallel to the axis of the mould core, but instead meets the inclined part of the outer surface of the cavity which forms a flank of the channel defined by the annular groove.  In this way, a pointed front edge on the annular ring is completely avoided.