An injection molding apparatus with hot runners, comprising: - a rear plate, which supports at least one main manifold with hot runners and is crossed by a plurality of air channels which communicate at least partly with a first face of said rear plate; - a manifold plate, which is fastened to said rear plate and has a plurality of nozzles configured to inject a stream of molten plastic material in a respective molding cavity, said manifold plate supporting a plurality of secondary manifolds with hot runners which are configured to feed respective groups of said nozzles with said stream of molten plastic material and communicate with the air channels of the main manifold, said secondary manifolds being accommodated in recesses of said manifold plate which are open toward said rear plate; - at each nozzle of each nozzle group, a valve stem for controlling said flow of molten plastic material through said nozzle, in which at least one part of said valve stem passes through the secondary manifold which feeds the respective nozzle group; - for each valve stem, an actuator which has a piston cylinder and a piston which can slide hermetically within said piston cylinder and is connected to said valve stem in order to open and close the respective nozzle; characterized in that: - said first face of the rear plate is in direct contact with a first face of the manifold plate substantially on the entire flat surface of the first face of the manifold plate which is not affected by said recesses of the secondary manifolds; - said actuators (26, 27) are completely external to the rear plate, are completely inside said recesses of the manifold plate and are arranged so that the respective piston cylinder is compressed between the first face of said rear plate and the secondary manifold which is crossed by the respective valve stem of the actuator and communicates with at least one of the air channels of said rear plate. - a rear plate, which supports at least one main manifold with hot runners and is crossed by a plurality of air channels which communicate at least partly with a first face of said rear plate; - a manifold plate, which is fastened to said rear plate and has a plurality of nozzles configured to inject a stream of molten plastic material in a respective molding cavity, said manifold plate supporting a plurality of secondary manifolds with hot runners which are configured to feed respective groups of said nozzles with said stream of molten plastic material and communicate with the air channels of the main manifold, said secondary manifolds being accommodated in recesses of said manifold plate which are open toward said rear plate; - at each nozzle of each nozzle group, a valve stem for controlling said flow of molten plastic material through said nozzle, in which at least one part of said valve stem passes through the secondary manifold which feeds the respective nozzle group; - for each valve stem, an actuator which has a piston cylinder and a piston which can slide hermetically within said piston cylinder and is connected to said valve stem in order to open and close the respective nozzle; - said first face of the rear plate is in direct contact with a first face of the manifold plate substantially on the entire flat surface of the first face of the manifold plate which is not affected by said recesses of the secondary manifolds; - said actuators (26, 27) are completely external to the rear plate, are completely inside said recesses of the manifold plate and are arranged so that the respective piston cylinder is compressed between the first face of said rear plate and the secondary manifold which is crossed by the respective valve stem of the actuator and communicates with at least one of the air channels of said rear plate.