a clamping frame for a plastic sheet thermoforming apparatus and which is designed for gripping and retaining the peripheral edge of the plastic sheet . the frame includes a pair of pivotally interconnected jaws which are pivoted between open and closed positions by an air cylinder actuator . the actuator is mounted to one of the jaws , and the operative pressurized air is fed to and removed from the actuator via fluid delivery lines which are located within the interior of the jaw to which the actuator is mounted . external air hoses are thereby avoided , and the air streams which flow through the jaw have a cooling effect .