A mold stack for defining a molding cavity for molding a preform, the mold stack comprising: a lock ring defining a tapered female seat; a split mold insert defining a molding surface of the molding cavity for molding a neck finish portion of the preform, the split mold insert having a split tapered male portion configured to cooperate with the tapered female seat of the lock ring to align and hold closed the split mold insert, the split mold insert further defining a pocket that extends coaxially through the split tapered male portion; a core ring defining a molding surface of the molding cavity for molding at least a portion of a top sealing surface of the preform, the core ring configured to be received within the pocket defined in the split mold insert; and a vent for evacuating air from the molding cavity, the vent including: a primary vent defined between the split mold insert and the core ring; and a secondary vent defined between the split mold insert and the core ring, characterised in that the primary vent comprises a gap, between the split mold insert and the core ring, that is substantially orthogonal to an axis of operation of the mold stack and the secondary vent comprises a gap, between the split mold insert and the core ring, that is substantially parallel to an axis of operation of the mold stack such that the primary vent may be widened while the width of the secondary vent remains substantially constant when the mold stack is placed into a cleaning configuration, the secondary vent acting as a vent for air without allowing any substantial amount of melt to pass therethrough when the mold stack is in the cleaning configuration. a lock ring defining a tapered female seat; a split mold insert defining a molding surface of the molding cavity for molding a neck finish portion of the preform, the split mold insert having a split tapered male portion configured to cooperate with the tapered female seat of the lock ring to align and hold closed the split mold insert, the split mold insert further defining a pocket that extends coaxially through the split tapered male portion; a core ring defining a molding surface of the molding cavity for molding at least a portion of a top sealing surface of the preform, the core ring configured to be received within the pocket defined in the split mold insert; and a vent for evacuating air from the molding cavity, the vent including: a primary vent defined between the split mold insert and the core ring; and a secondary vent defined between the split mold insert and the core ring, characterised in that the primary vent comprises a gap, between the split mold insert and the core ring, that is substantially orthogonal to an axis of operation of the mold stack and the secondary vent comprises a gap, between the split mold insert and the core ring, that is substantially parallel to an axis of operation of the mold stack such that the primary vent may be widened while the width of the secondary vent remains substantially constant when the mold stack is placed into a cleaning configuration, the secondary vent acting as a vent for air without allowing any substantial amount of melt to pass therethrough when the mold stack is in the cleaning configuration. a primary vent defined between the split mold insert and the core ring; and a secondary vent defined between the split mold insert and the core ring, characterised in that the primary vent comprises a gap, between the split mold insert and the core ring, that is substantially orthogonal to an axis of operation of the mold stack and the secondary vent comprises a gap, between the split mold insert and the core ring, that is substantially parallel to an axis of operation of the mold stack such that the primary vent may be widened while the width of the secondary vent remains substantially constant when the mold stack is placed into a cleaning configuration, the secondary vent acting as a vent for air without allowing any substantial amount of melt to pass therethrough when the mold stack is in the cleaning configuration.