A preform molding system having a cavity half mountable to a stationary platen of an injection molding machine, and a core half mountable to a moving platen of the injection molding machine, the preform molding system for creating preforms that are blow molded into a final article, the preform molding system comprising: a mold stack assembly having a cavity portion and a core portion, the cavity portion coupled to the cavity half and including a cavity insert, and the core portion coupled to the core half and including a core insert, a pair of neck rings (370A, 370B), and a stripper ring, the core insert having an undercut that defines an annular pro-trusion on an internal surface of a preform created in the mold stack assembly, wherein the undercut includes an inward portion that slopes towards a central axis (A C) of the mold stack assembly, and an outward portion that slopes away from the central axis (Ac), and wherein the inward portion slopes towards the central axis (Ac) of the mold stack assembly from a calibration portion section of the core insert, and wherein the preform molding system is configured to retract the pair of neck rings (370A, 370B) away from the core insert prior to stripping the preform from the core insert via the stripper ring, characterized in that the outward portion is a curved surface to which the inward portion is tangent, and the outward portion has an arcuate cross-sectional shape that extends between the inward portion and a molding surface of the stripper ring. a mold stack assembly having a cavity portion and a core portion, the cavity portion coupled to the cavity half and including a cavity insert, and the core portion coupled to the core half and including a core insert, a pair of neck rings (370A, 370B), and a stripper ring, the core insert having an undercut that defines an annular pro-trusion on an internal surface of a preform created in the mold stack assembly, wherein the undercut includes an inward portion that slopes towards a central axis (A C) of the mold stack assembly, and an outward portion that slopes away from the central axis (Ac), and wherein the inward portion slopes towards the central axis (Ac) of the mold stack assembly from a calibration portion section of the core insert, and wherein the preform molding system is configured to retract the pair of neck rings (370A, 370B) away from the core insert prior to stripping the preform from the core insert via the stripper ring, characterized in that the outward portion is a curved surface to which the inward portion is tangent, and the outward portion has an arcuate cross-sectional shape that extends between the inward portion and a molding surface of the stripper ring. wherein the undercut includes an inward portion that slopes towards a central axis (A C) of the mold stack assembly, and an outward portion that slopes away from the central axis (Ac), and wherein the inward portion slopes towards the central axis (Ac) of the mold stack assembly from a calibration portion section of the core insert, and wherein the preform molding system is configured to retract the pair of neck rings (370A, 370B) away from the core insert prior to stripping the preform from the core insert via the stripper ring, characterized in that the outward portion is a curved surface to which the inward portion is tangent, and the outward portion has an arcuate cross-sectional shape that extends between the inward portion and a molding surface of the stripper ring.