Abstract: A blow molding machine wherein a shield is movable to and from an operative position in which it prevents access to the mold sections. The hydraulic motors which move the mold sections can receive pressurized fluid only when the shield is locked in the operative position by a roller. When the roller is retracted from its arresting position, a mechanical power train automatically closes valves which are installed in conduits connecting the motors with oil pumps. The valve elements of the valves are mechanically coupled to each other and to the roller, and are biased by a spring which tends to maintain the roller in the arresting position. A single-acting hydraulic cylinder is employed to move the roller from the arresting position. The shield can be moved by hand and/or by a double-acting hydraulic cylinder.