Abstract: A method of forming a flexible elastomer polymer fluid jacket for a plastic processing mold, and a novel elastomer polymer fluid jacket. The method comprises attaching channel formers to the rear surface of the metal shells of the mold sections to define a desired flow configuration, securing a support casting to the rear surface of the metal shells spaced from the channel formers to define a designed cavity, feeding a curable elastomer polymer typified by silicone elastomers into the cavity, curing the elastomer polymer, and removing the channel formers whereby the cured elastomer polymer forms a fluid jacket. A mold having mold components with flexible polymer fluid jackets between the mold shells, preferably nickel shells, and support castings permit enhanced heat transfer to the mold cavity while providing improved thermal insulation to the mold. The elastomer polymer also enables multiple modular configurations of the heating and/or cooling function of the mold.