Apparatus for injection molding

An apparatus for injection molding using a stack mold comprising first and second mold assemblies interposed between a movable platen and a stationary platen comprises at least one centering device for coupling movement of the movable platen to intermediate mold members of the mold assemblies, the centering device comprising first and second helical gear mechanisms and a transmission coupling rotation of helical gear segments of the first and second helical gear mechanisms. Effective pitch of the helical gear mechanisms and the ratio of rotation coupled by the transmission are chosen to maintain the intermediate mold members centered in or offset from center in the separation between the movable platen and stationary platen.