A conditioned jack for actuating a valve pin of an injector for apparatus for injection molding of plastic materials, includes a cylinder having at least one thrust chamber delimited on one side by a first bottom and on the other side by a second bottom, a support of the cylinder for securing the jack to the molding apparatus, and a flow circuit for a conditioning fluid of the jack, which is formed within a bushing which constitutes said second bottom, is separable from the cylinder and is provided with inner and outer annular sealing gaskets of respectively the cylinder and a stem.