A mold stack (103, 303) comprising: two adjacent mold stack components (114, 106, 314, 306) for collectively defining at least a portion of a molding cavity (105, 305); the two adjacent mold stack components being a split mold insert (114, 314) and a cavity insert (106, 306); a vent (150, 350), at least partially defined by one of the mold stack components (114, 314), that is adjustable between: a molding configuration wherein the vent is configured to vent gas from the molding cavity while preventing passage of any substantial amount of melt therethrough; and a cleaning configuration wherein the vent is configured to receive melt from the molding cavity for cleaning the vent; and characterised by a junction (144, 344) defined between two respective mating faces (133, 142, 333, 342) of the two mold stack components, the junction comprising a tongue and groove interface and being adjustable between: a molding configuration wherein the mating faces contact one another to define a parting line of the molding cavity; and a cleaning configuration wherein the mating faces are separated to create a space (146, 346) between the mating faces, the space acting as an extension of the molding cavity, and wherein the junction further defines an auxiliary melt barrier (151, 351) for preventing uncontrolled flashing from the extension. two adjacent mold stack components (114, 106, 314, 306) for collectively defining at least a portion of a molding cavity (105, 305); the two adjacent mold stack components being a split mold insert (114, 314) and a cavity insert (106, 306); a vent (150, 350), at least partially defined by one of the mold stack components (114, 314), that is adjustable between: a molding configuration wherein the vent is configured to vent gas from the molding cavity while preventing passage of any substantial amount of melt therethrough; and a cleaning configuration wherein the vent is configured to receive melt from the molding cavity for cleaning the vent; and characterised by a junction (144, 344) defined between two respective mating faces (133, 142, 333, 342) of the two mold stack components, the junction comprising a tongue and groove interface and being adjustable between: a molding configuration wherein the mating faces contact one another to define a parting line of the molding cavity; and a cleaning configuration wherein the mating faces are separated to create a space (146, 346) between the mating faces, the space acting as an extension of the molding cavity, and wherein the junction further defines an auxiliary melt barrier (151, 351) for preventing uncontrolled flashing from the extension. a molding configuration wherein the vent is configured to vent gas from the molding cavity while preventing passage of any substantial amount of melt therethrough; and a cleaning configuration wherein the vent is configured to receive melt from the molding cavity for cleaning the vent; and characterised by a junction (144, 344) defined between two respective mating faces (133, 142, 333, 342) of the two mold stack components, the junction comprising a tongue and groove interface and being adjustable between: a molding configuration wherein the mating faces contact one another to define a parting line of the molding cavity; and a cleaning configuration wherein the mating faces are separated to create a space (146, 346) between the mating faces, the space acting as an extension of the molding cavity, and wherein the junction further defines an auxiliary melt barrier (151, 351) for preventing uncontrolled flashing from the extension. a molding configuration wherein the mating faces contact one another to define a parting line of the molding cavity; and a cleaning configuration wherein the mating faces are separated to create a space (146, 346) between the mating faces, the space acting as an extension of the molding cavity, and wherein the junction further defines an auxiliary melt barrier (151, 351) for preventing uncontrolled flashing from the extension.