Disclosed herein, amongst other things is a first stack portion (112, 212) of a mold stack (110, 210) for use in an injection mold (100, 200). The first stack portion (112, 212) includes a first stack member that is configured to define a molding cavity (116, 216) in cooperation with a second stack portion (114, 214) of the mold stack (110, 210), a second stack member that is movable relative to the first stack member, and 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 relative movement of the first stack member and the second stack member.