A first stack portion (112, 212) of a mold stack (110, 210) for use in a mold (100, 200), a molding cavity (116, 216) is definable between the first stack portion (112, 212) and a second stack portion (114, 214) of the mold stack (110, 210) that are associated with a first mold half and the second mold half of the mold (100, 200), respectively, the first stack portion (112, 212), comprising: a first stack member that is configured to define the molding cavity (116, 216) in cooperation with the second stack portion (114, 214) of the mold stack (110, 210); a second stack member that is movable relative to the first stack member; the first stack member and the second stack member comprise a core insert (113, 213) and a stripper sleeve (140, 240), respectively, wherein the stripper sleeve (140, 240) is slidably arranged around the core insert (113, 213), the core insert (113, 213) being configured to define an inner portion the molding cavity (116, 216), the stripper sleeve (140, 240) being configured to strip a molded article from the core insert (113, 213) with relative movement thereof; the core insert (113, 213) comprises an inner core (120, 220) and an outer core (130, 230), the outer core (130, 230) being slidably arranged around the inner core (120, 220), and wherein an air channel (134, 234) is defined between the outer core (130, 230) and the inner core (120, 220); characterized in that an air valve (160, 260) is defined between the first stack member and the second stack member, the air valve (160, 260) being selectively actuatable with the relative movement of the first stack member and the second stack member, the air valve (160, 260) is configured to control air flow between an air source (196, 296) and the air channel (134, 234); and an exhaust valve (161, 261) is defined between the first stack member and the second stack member, the exhaust valve (161, 261) being selectively actuatable with the relative movement of the first stack member and the second stack member the exhaust valve (161, 261) is configured to control the air flow between the air channel (134, 234) and a vent channel (138, 238). a first stack member that is configured to define the molding cavity (116, 216) in cooperation with the second stack portion (114, 214) of the mold stack (110, 210); a second stack member that is movable relative to the first stack member; the first stack member and the second stack member comprise a core insert (113, 213) and a stripper sleeve (140, 240), respectively, wherein the stripper sleeve (140, 240) is slidably arranged around the core insert (113, 213), the core insert (113, 213) being configured to define an inner portion the molding cavity (116, 216), the stripper sleeve (140, 240) being configured to strip a molded article from the core insert (113, 213) with relative movement thereof; the core insert (113, 213) comprises an inner core (120, 220) and an outer core (130, 230), the outer core (130, 230) being slidably arranged around the inner core (120, 220), and wherein an air channel (134, 234) is defined between the outer core (130, 230) and the inner core (120, 220); characterized in that an air valve (160, 260) is defined between the first stack member and the second stack member, the air valve (160, 260) being selectively actuatable with the relative movement of the first stack member and the second stack member, the air valve (160, 260) is configured to control air flow between an air source (196, 296) and the air channel (134, 234); and an exhaust valve (161, 261) is defined between the first stack member and the second stack member, the exhaust valve (161, 261) being selectively actuatable with the relative movement of the first stack member and the second stack member the exhaust valve (161, 261) is configured to control the air flow between the air channel (134, 234) and a vent channel (138, 238).