an injection molding machine for forming a multi - layer plastic article by over molding where the second layer of the article includes a portion having a different geometrical profile than the first . the injection mold machine comprises an array of one or more cores which engage arrays of one or more first cavity and arrays of one or more composite cavities . each composite cavity is formed from the combination of a second cavity and a cavity extension which carries at least a portion of the different geometrical profile . in one embodiment , the cavity extension comprises a pair of cavity portions which are mounted adjacent the core to laterally moveable slides on a movable platen . in another embodiment , the cavity extension is a single element which is moved between a disengaged position wherein the core can be inserted into the first cavity and an engaged position wherein the core is inserted into the composite cavity .